Hard-disk drives, or HDDs, are mechanical devices. They work by recording data on a spinning magnetic platter or platters. By contrast, solid-state drives are made of chips and have no moving parts. They are close cousins to the so-called flash memory used in digital cameras, cellphones and smaller-capacity music players. They record data to special memory chips that retain their contents even when the device is turned off.

Solid-state drives have some key advantages. Because they lack moving parts, they are faster, draw less power, are harder to damage and are quieter than hard drives. Unfortunately, today’s early versions of SSDs for laptops also have two big drawbacks when compared with hard drives: They offer much lower capacity and have much higher prices.

For instance, on the newly announced Apple MacBook Air ultrathin laptop, the HDD version costs $1,799 and stores 80 gigabytes. The SSD version costs $2,798, but actually stores less — just 64 gigabytes. On the Toshiba Portege R500 subnotebook, the basic hard-drive version costs $1,999 and stores 120 gigabytes. The cheapest SSD version is $2,699 and also stores just 64 gigabytes.

Despite these limitations, I believe SSDs are likely to become more common and more popular as their capacities increase and their prices drop. Samsung, which makes the 64-gigabyte SSDs in both the Apple and the Toshiba, has already announced an SSD with twice the capacity that costs much less per gigabyte of storage.

I’ve been testing the SSD models of the Portege R500 and the MacBook Air to see how they measure up to their HDD counterparts. My verdict is that the SSD does deliver on its promises, but, in some cases, just barely.

For the small slice of users who are deeply and constantly worried about hard-disk failures, it may be worth it to pay a huge premium today for an SSD that stores less. Because SSDs aren’t subject to mechanical failures, your data are probably safer on them. But for mainstream users, my conclusion is that it’s too early to take the plunge on SSDs, and the best strategy is to wait for prices to drop sharply and for capacity to rise.

In my tests, I focused solely on comparing the hard-drive and SSD models of each machine, which I had had already reviewed in earlier columns. On the same computer, I wondered, would the SSD make a significant difference in speed and in battery life?

To measure battery life, I conducted my usual harsh test, where I turn off all power-saving software, set screen brightness to maximum, turn on the Wi-Fi and play an endless loop of music.

In this test, the SSD made little difference in the MacBook Air and, in fairness, Apple is making no claims of any significant battery-life gains on its SSD model. The SSD MacBook gave me just five more minutes of battery life. Apple says this is because its hard-drive model already uses a very low-power drive.

On the Portege R500, my first battery test with the SSD model actually yielded significantly less battery life than the hard-drive model. The reason: Toshiba ships the base SSD model with a battery with only half of the capacity of the hard-drive model.

When I swapped in the normal battery, which costs $117 extra, the SSD model gave me an added 1 hour and 21 minutes of battery life, about a 36% increase. That extra battery life likely would translate to nearly 2½ hours in more normal usage. It may be worth the huge price premium for some folks.

On both computers, the SSD was faster than the HDD models. The SSD version of the Apple booted up from a cold start, and rebooted with several programs running, about 40% faster. But the gain isn’t as impressive as it seems because even the hard-drive versions of the MacBook Air booted up in under a minute and rebooted in just a little over a minute.

On the Toshiba, which was running Windows XP, the SSD model knocked about 40 seconds off a cold boot time on the HDD version of 2 minutes and 7 seconds. On my reboot test, starting with several programs running, the SSD model was 80 seconds faster. I imagine that on laptops with the slow-booting Windows Vista, the improvements might be more meaningful.

I also tested launching Microsoft Word and Excel, and opening a couple of hefty PDF files on both machines. The SSD versions were faster. But in most cases, the gains were just a few seconds or even fractions of a second.

All in all, the SSD is a promising improvement over the hard drive, but now is not the time for most users to buy it.

Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.

Unfortunately, every attempt at this concept has included too many compromises to justify their often surprisingly high prices. I’ve been testing the latest effort at such a device, the Eee PC, from a Taiwan-based laptop maker called Asus. It does better than some of the earlier contenders in certain respects, such as text entry and price. But it still is likely to prove unsatisfying for many road warriors.

Asus doesn’t even call the Eee a computer, referring to it as a “mobile Internet gadget.” Instead of using Microsoft Windows as its operating system, the Eee uses a specially designed version of the open Linux operating system, and comes preloaded with a variety of open-source programs for Web browsing, performing office tasks, playing music and videos, running games and managing photos.

The Eee has a much smaller footprint than even the subnotebook category of laptop, such as the much-publicized MacBook Air unveiled by Apple this week (which I’ll review after I have thoroughly tested it), or subnotebooks from Sony and Lenovo. It weighs a mere two pounds, is just under 9 inches wide and just over 6 inches deep. It is thicker than the new Apple and some other subnotebooks, ranging from 0.79 inches at its thinnest point to 1.26 inches at its thickest. The overall effect is small, but stubby.

The Eee’s price is only a fraction of what typical subnotebooks cost — from $300 to $500, depending on configuration. The model I tested, called the Surf, is the base $300 entry. With its pastel blue lid, and tiny size, it looks like something Barbie might use. But it can perform real work, even though it comes with only 512 megabytes of memory and a scant two gigabytes of storage space.

One reason the device costs and weighs so little is that there is no hard disk. Files are stored on memory chips. It is possible to add storage by popping in a flash memory card or by connecting a USB drive to one of the three USB ports.

Unlike some computers in its category, the Eee isn’t a tablet. It takes the clamshell form of a traditional laptop and, when opened, reveals a full, if very cramped, keyboard. By including a real keyboard and charging so little, Asus has overcome two of the problems that plagued Samsung’s Q1 ultramobile computer awhile back.

Asus Eee PC

The user interface on the Eee is simple and clear. It consists of tabs labeled Internet, Work, Learn, Play, Settings and Favorites. Each tab contains large, colorful icons. For instance, the Work tab includes icons for Documents, Spreadsheets and Presentations. These tabs lead to various modules of the free OpenOffice suite, a competitor to Microsoft Office.

The Internet tab has various icons, such as Web Mail, Web and Wikipedia, that open the Firefox Web browser. You get to this tabbed screen by just pressing a Home button.

In my tests of the Eee, I was able to use all manner of Web sites, send and receive Web-based email, compose and open Microsoft Word documents from other computers, play music, and view photos.

So, with a low price, a small size, a real keyboard, and a clean user interface, what’s not to like about the Eee?

Well, for starters, its tiny 7-inch display is just too stingy for serious work. You can make up for a small screen size with high resolution, but the 800 by 480 resolution on the Eee — which can’t be changed — is so wimpy that very few lines of text can be seen at any one time. This means you’ll have to do so much scrolling, it’s likely to drive you crazy.

Also, the lack of a hard disk, and the relative paucity of truly simple Linux software, means that most nontechie users will be stuck with the included programs, which can’t be removed and which don’t include a calendar or contacts program, or even an email program. You can only use Web-based email.

And just below the Eee’s simplified user interface lurks the complicated terminology and software design characteristic of Linux. Some error messages I saw were indecipherable.

Beyond that, many of the Linux programs included were far cruder and harder to figure out than, say, Microsoft’s Photo Gallery or Apple’s iTunes. The Eee wouldn’t automatically reconnect to a known wireless network, and it wouldn’t recognize my Kodak digital camera.

Some of these problems can be solved if you install Microsoft Windows XP on the Eee. Asus includes instructions on doing so. But you have to supply your own copy of Windows.

The Eee is a valiant effort, but it still has too many compromises to pry most travelers away from their larger laptops.

Email me at mossberg@wsj.com. Find all my columns and videos online, free, at the new All Things Digital Web site, http://walt.allthingsd.com.