Netbooks still constitute a smaller niche than laptops and the exploding smart phone, or hand-held computer, category. But they are threatening to break into the mainstream in a big way, especially in an economic climate where a low price and fewer bells and whistles are suddenly more attractive.

They are much more portable than most standard laptops. They are easier to use on a plane or carry around town. And they are way cheaper, between $300 and $500, than the very lightest, thinnest standard laptops, which often top $1,000.

Compared with even an amazingly powerful pocket device, like the iPhone, the Google (GOOG) G1, or the forthcoming BlackBerry Storm, a netbook, at about twice the price, offers a much larger keyboard and screen. And they can run far more sophisticated software and perform a much wider variety of computing tasks.

But netbooks come with serious compromises. While they are great for light use on the go, their cramped screens and keyboards, and slow processors, make them much less potent and less comfortable to use than even a so-called ultraportable laptop. And, as small as they are, they can’t fit in a pocket like smart phones can, be as easily used as a still camera, or function as a cellphone.

Netbooks aren’t tablets. They look and act like regular clamshell-style laptops with keyboards and track pads, but are much smaller. Most current models have 8.9-inch screens, though some now sport 10-inch displays. Dell (DELL) is even planning soon to launch a netbook with a 12-inch screen for around $600, which will blur the line with traditional laptops, some of which can be bought for less with larger displays.

Compared with sleek, thin, but much costlier notebooks, such as the Apple (AAPL) MacBook Air or the Lenovo ThinkPad X300, the netbooks of today are stubbier and chunkier. But they take up much less room on an airline tray table. When the person in front of you reclines, you can happily keep using them, because their screens are so much smaller and extend upward so much less.

The early netbook models relied on the unfamiliar and somewhat geeky Linux operating system, and most still offer it as an option. But many now also can be purchased with Windows XP, with which consumers are far more experienced, and which can run many more well-known programs.

At the start of this year, most netbooks lacked hard disks, instead offering very limited storage via memory chips — often less storage than a $199 iPhone. They were pitched as limited devices mainly meant for using the Internet — thus the name “netbook” — and their makers assumed users mainly would use Web-based applications.

Now, many offer decent-size hard disks and include serious programs, such as Microsoft (MSFT) Office or Microsoft Works. But none offers a built-in DVD drive, which makes it hard to install some new software.

To offer readers a feel for today’s netbooks, I selected four representative models to test and review. I am not declaring these four as the best on the market, nor do I mean to slight makers like H-P, whose entries aren’t included in this review. The truth is, there are far more similarities than differences among competing netbooks that might make one model stand out from the others.

All four of the models I tested use Intel’s new low-power Atom processor. All have decent screen resolution — much better than the original Asus. But none can display a full Web page, or even most of a Web page, without scrolling. Each has three USB ports.

Three of the four have good battery life, but getting good power in most models means using a larger battery that adds weight and bulk.

All of my test models ran XP, not Linux, because I believe that’s the better choice for average consumers.

Here are minireviews of these four netbooks.

Acer Aspire One: The $349 blue Acer One weighs a little over two pounds with its standard battery, and has a bright, sharp 8.9-inch screen. It comes with a 120-gigabyte hard disk and 1 gigabyte of memory. It’s a little over an inch thick, and its footprint is much smaller than that of a standard sheet of paper.

As on all the other models, I tried a word processor, either Microsoft Word or Works, and several popular non-Microsoft programs: Adobe Reader, Apple’s iTunes and Mozilla’s Firefox browser. The Acer handled all of them well, though, as with all the other netbooks I tried, its speakers are mediocre.

The Acer’s small keyboard is very nicely done. Its keys are large enough, and separated and sculpted enough, to make typing comfortable and accurate, though I wouldn’t want to write a novel on it.

But the Acer has two big drawbacks. Its battery life is miserable. On my tough battery test, where I turn off all power-saving features, crank up the screen brightness, turn on the Wi-Fi, and play a continuous loop of music, it couldn’t even squeeze out two hours. In normal use, that might mean 2½ hours. To fix that problem, you can spend $50 more on a version with a double-size battery, at the cost of added weight and bulk. This costlier version also boosts the hard disk to 160 gigabytes.

The other problem, which can’t be fixed with any factory options, is that the Acer One has a terrible track pad. It’s too cramped vertically for comfortable use, and the buttons, which are mere slivers, are arranged on the sides instead of below the pad. Moving the cursor or selecting text is awkward and inaccurate.

Lesser problems are that the Acer includes only the older, slower, “G” flavor of Wi-Fi and a low-resolution Webcam.

The Dell Mini 9 has an 8.9-inch screen, is compact and has a big battery, but it lacks a hard disk and offers just 8 gigabytes of flash memory.

Dell Mini 9: Like the less costly Acer, the $399 Dell Mini has an 8.9-inch screen, but it’s a bit narrower horizontally. Its standard battery is larger, making it slightly heavier but still very light.

The Dell is a throwback to the older concept of netbooks. It lacks a hard disk and offers just 8 gigabytes of flash memory, plus 2 gigabytes of free online storage. It has just half a gigabyte of memory. For extra money, you can double the flash storage and memory.

Because of its bigger battery, and its lack of a power-sucking hard disk, the Dell beat the Acer handily in my battery test, getting just under three hours, which means that, in normal use, you would likely see four hours.

The Dell has the same wimpy Wi-Fi and Webcam as the Acer. But its track pad, while small, is much larger vertically and easier to use, with buttons where you expect to find them. It ran all my test software OK.

However, the Dell had by far the worst keyboard in my test group. Because of its compact width, the tab, arrow and other keys are squeezed to a ridiculously narrow size that impedes typing.

MSI Wind U100: This is a $399 machine (after a recent price cut) with a 10-inch screen, and comes from a Taiwan company better known in the U.S. for making computer components than entire computers. The model I tested, with a double-size battery, is $429. My test unit was white, weighed a tad over three pounds, and had 1 gigabyte of memory and a 160-gigabyte hard disk.

Despite the larger screen, the Wind still fits very well on a cramped airline tray, and it has a well-designed keyboard. It comes with a button that can slow down or speed up the processor to save battery life or add oomph. And there’s a function that can magnify portions of text.

It has a standard, decent Webcam and can use the newest “N” flavor of Wi-Fi. It ran all my test software just fine.

With my test model’s bigger battery, which protrudes from the bottom, the Wind did very well on my test at its standard processor speed, lasting three hours and 37 minutes. That suggests you could get four to five hours in normal use. Presumably, the standard model with the smaller battery would get half of that life, though you could stretch it by stepping down the processor speed.

Overall, I liked the MSI Wind a lot. My only real gripe is that the track pad is small and has only a single thin button, which performs a left or right click. This button is too small and sluggish for optimal use.

Asus Eee 1000H: Asus, another Taiwan company known as a component maker, is the king of netbooks. In fact, it has so many different, and frequently changing, netbook models that its product lineup can be a blur. The one I tested has a 10-inch screen and costs $475, making it the costliest netbook in this group. It’s also the heaviest, edging out my test Wind slightly.

Like the Wind, my Eee 1000H had a large battery that protruded from the bottom. It doesn’t come with a smaller battery. Also like the Wind, it has a standard Webcam, the faster “N” Wi-Fi, and a 160-gigabyte hard disk with 1 gigabyte of memory.

The keyboard on the 1000H was the best of this lot, with well-designed keys. It also had the roomiest and most functional track pad, though its buttons — integrated with a metal border around the track pad — took some getting used to.

The Asus, like the Wind, has the capability to tweak the speed of its processor. It also has a button that can change the screen resolution, though I found that the nonstandard resolutions looked distorted.

In my battery test, at its standard processor speed, the 1000H got three hours and 32 minutes, suggesting that in normal use it could deliver between four and five hours — more if you use the lower processor speed.

The Asus handled all my test software well. It comes with a greater variety of built-in programs than the others and offers 20 gigabytes of free online storage.

Bottom line: If you want a machine for light use, a light price and a light weight, a netbook is waiting and is worth a try. Just don’t expect the same experience as on a standard laptop or the convenience of a smart phone.

Find all of Walt Mossberg’s columns and videos online, free, at the All Things Digital Web site, walt.allthingsd.com. Email him at mossberg@wsj.com.