Amazon's Kindle Fire "offers a disappointingly poor user experience," according to usability expert Jakob Nielsen.

Nielsen, who has a PhD in human-computer interaction, studied the behaviour of four Kindle Fire users. Although that's a tiny sample, Nielsen argues that small, qualitative studies offer more insight than bigger studies focused on metrics.

The Kindle Fire hardware is too heavy, and unpleasant to hold for long periods at a time, Nielsen observed. "Unless you have forearm muscles like Popeye, you can't comfortably sit and read an engaging novel all evening."

Software issues

Granted, this is true of most 7in tablets. And although some lighter tablets are available, such as Samsung's Galaxy Tab 7.0 Plus, we didn't find the Fire particularly tiresome in our review. Nielsen also derided the Kindle Fire's lack of physical buttons for turning e-book pages, but again, that's true of every tablet on the market (and I don't think it's as frustrating as Nielsen makes it out to be).

The bigger issue, according to Nielsen, is software. The Kindle Fire interface has some trouble spots, like buttons that are too small and screen updates that are too slow. Indeed, sluggish performance was one of our main gripes.

Nielsen also reserved some scorn for apps and websites, as viewed on the Kindle Fire. Mistaken taps were common among users, he noted. Full-sized websites were hard to read and interact with, and mobile-optimised sites are better suited for the 7in display.

Commercial success

If Kindle Fire users will kindly put down their pitchforks for a moment, I think there's a good point that Nielsen is trying to make here: you can't shoehorn the interface of a smartphone app or a 10in tablet app into a 7in display. "A 7in tablet is a sufficiently different form factor that it must be treated as a new platform," Nielsen writes.

Until now, app and website developers haven't had any good reason to design for 7in displays. The good news is that Amazon's Kindle Fire is becoming a bigger commercial success than any other non-iPad tablet, so we may see more developers catering to 7in screens. I don't think users should be satisfied with viewing mobile-optimised websites on a small tablet, especially because so many mobile sites are awful.

And before you get too upset about Nielsen's findings, keep in mind that finding usability weak points is his job. He found plenty of nits to pick with Apple's iPad as well.