The 7-inch Kindle Fire is easier to slip into a purse or large pocket than the 10-inch iPad.
Editor's note: Amy Gahran writes about mobile tech for CNN.com. She is a San Francisco Bay Area writer and media consultant whose blog, Contentious.com, explores how people communicate in the online age.
(CNN) -- Although it has been on the U. S. market for just three weeks, Amazon's Kindle Fire is expected to become the second-most popular tablet in the world by the end of this year, according to the research firm IHS.
But the device is not having a very good week.
There have been complaints on Amazon community forums by some Kindle Fire users who are experiencing problems accessing Wi-Fi networks and the Internet.
And Web usability expert Jakob Nielsen posted a fairly scathing review on his site Monday of the hot-selling tablet.
A few lowlights:
Kindle Fire: Good, but no iPad killer 
Kindle Fire: Better than iPad? 
Tablet wars heat up with Kindle Fire "Using the Web with the Silk browser is clunky and error-prone. Reading downloaded magazines is not much better."
"The Fire is a heavy object. It's unpleasant to hold for extended periods of time. Unless you have forearm muscles like Popeye, you can't comfortably sit and read an engaging novel all evening. The lack of physical buttons for turning the page also impedes on the reading experience for fiction. On the older Kindles, it's easy to keep a finger on the button when all you use it for is to turn the page. In contrast, tapping an area of the screen disrupts reading enjoyment, is slightly error-prone, and leaves smudges on the screen. The Fire screen also has more glare than the traditional Kindle."
"The magazine reading experience could be good but actually is miserable. ... Many magazines don't have a 'home page' where users can return after finishing an article. Headlines on magazine covers aren't clickable. ...'Page View' is unreadable and 'Text View' has the worst layout I've seen in years."
"If I were given to conspiracy theories, I'd say that Amazon deliberately designed a poor Web browsing user experience to keep Fire users from shopping on competing sites. Amazon's own built-in shopping app has great usability, so they clearly know how to design for the tablet."
All Nielsen's criticisms may be true. But they also may miss the point, from a consumer perspective: The cheapest iPad 2 costs $500. The Kindle Fire costs $200. And it's still a rough economy out there.
Also, there's portability: The 7-inch Kindle Fire is easier to slip into a purse or large pocket than the 10-inch iPad.
GigaOm's Colin Gibbs observed about Nielsen's critique: "When it comes to new tablets, I think we all suffer from a case of heightened expectations due to the fantastic iPad. The Kindle Fire is inferior to Apple's tablet, to be sure, but it's much smaller and much cheaper -- so that should be no surprise. Other Kindle Fire reviews have been much more positive, and I think the device will find a big audience of consumers willing to sacrifice some performance and size for the more attractive price."
CNN reporter Mark Millian's detailed Kindle Fire review deemed the tablet "a worthwhile bargain."
Smaller tablets might have a huge impact on the consumer market not because they're top-notch technology, but because they can fill a crucial economic niche, especially for households that can't afford or don't want an iPad or another computer.
The opinions expressed in this commentary are solely those of Amy Gahran.
