After playing heavily with it for awhile, I can happily report one more critical quality the handheld 20Q has in spades: it's superbly well-designed. I'm something of a stickler for good design (I consider the iPod a masterpiece), and I have a low tolerance for cluttered, overly-complicated electronics. The 20Q has a very simple, clear interface, with all buttons accessible and none of them serving more than one purpose except the "Yes" button, which doubles as the "On" button when the thing's off and the "New Game" button when it's on. The display is a delightful scrolling red backlit screen; two buttons on the side speed up and slow down the scrolling, so you don't have to sit there with eyes glazing over while the questions crawl past. At the fastest scroll rate you can pretty much storm through a game quickly, but for younger players, you can slow the scroll down so they have time to read the questions.
In hours of playing with it I never once hit the wrong button. And - Praise the Design Gods! -- the annoying sound effects are easily muted with the touch of a button and they stay off until and unless you turn them back on - even when the 20Q is off (it shuts itself off after 60 seconds of inactivity).
Best of all, unlike the Web version, the handheld 20Q game breaks in every five or six questions for a taunt ("I can't believe you're thinking of that!") or a boast ("I know almost everything!"). These are mercifully quick, so not only don't they interrupt the flow of your game, but they actually add two important benefits. The first is that they give you a brief comic respite from having to think about your object and how to answer the questions (without which the game could begin to feel slightly like a test). Second, and most importantly, it "humanizes" the 20Q; you never really feel you're simply inputting data into a computer since it seems to have a personality. The fact that the chosen personality is a bit of a joker and a braggart makes it more fun when you stump it, and also makes it seem more personal when it nails you.