Nintendo's
Game Boy Advance (AGB-001) had actually been waiting in the wings for a number of years (since around
1997 at least) before it was released in
2001. Nintendo finally felt that they had gone as far as they could with the first generation
Game Boy family (which debuted in
1989 and subsequently became the best-selling games
console in history), and that they were able to produce and sell the devices cheaply enough to pass on the torch (the GBA actually cost rather less at launch than the original
Game Boy had done).
The machine is powered by a 32-bit RISC processor (as detailed in writeups above), and is significantly more powerful than the SNES (the misconception that it's merely a portable SNES probably stems from the reliance on remakes of 16-bit titles in the first wave of software). It is also compatible with over 99% of Game Boy and Game Boy Color titles, although the carts protrude from the top of the machine somewhat.
Sound and graphics capabilities are greatly superior to the original Game Boy and its contemporaries. The GBA is capable of performing sprite scaling and rotation with consummate ease (as I write, the technical showcase is a version of DooM ported from the Jaguar- it is unclear as to how well the machine can 'do' true polygons, and it has a fairly restrictive pool of memory). Sound is full multi-channel stereo, sounding uncannily like a SNES, and best experienced through headphones. In layman's terms, the machine is vastly more powerful than the SNES, but less powerful than the PSX (except for sprite-based games, where it is a shitload better).
The hardware design does have some shortcomings. The battery life is a mere 20 hours from 2 AA batteries (half that of the NeoGeo Pocket Color). The d-pad is small and unresponsive, and the shoulder buttons spongy and presumably leaf-switched (and very easy to accidentally hit if your hands are large). The start and select buttons are also inconveniently placed and very small, but this has been the case since the NES, and it has always mystified me why Nintendo still use them. In fact, most of the machine's problems are just instances where they've ignored the improvements made by the NeoGeo Pocket Color, for reasons of cost (and honour perhaps?).
The infamous 240x160 colour screen is not as dark as the more hysterical games sites have made out, but it does prevent the machine being used in less-than-great lighting conditions (ironically, a machine that is supposed to be portable makes a lot of demands on its environment- I usually play mine plugged in near to an overhead lamp). In the end, it's a trade-off between resolution and visibility, and I think that they couldn't sacrifice the resolution any further without seriously hobbling the machine. Suggesting that there should be a backlight built into the machine is unrealistic. In good light, the colours are rich and there is little or no motion blur.
Anyone considering buying a GBA should not be put off by the horror stories about the screen. The main consideration should be the games library, and as this is still growing every day, I can't make a final judgement. The one thing I can say with certainty is that you will be spoilt for choice.
If your main incentive is to play old games from a surprisingly broad spectrum (from old Amiga games to coin-ops, there's even a homebrewn ZX Spectrum emulator out there), you should probably ask whether you think it's worth stumping up the price of a new game for an old (even abandonware) title, usually with minimal improvements, just for the added bonus of being able to play it on the move.
There are an increasing number of original games that use the machine's full capabilities, and in my opinion these should be investigated first by the potential purchaser. Depending on your moral standpoint, you can pick up a lot of games cheaply by purchasing a backup device (which you can also use to legally play homebrew titles and demoes, of which there is a burgeoning scene).
You can also use the GBA as an 'intelligent' Nintendo GameCube controller (with the appropriate cable), or link up four GBAs for multiplayer games. This is a great feature, which is a damn sight easier than setting up a LAN, but be aware that unless you have friends with the same taste in games (and therefore enough carts for everyone), you will have to put up with long loading pauses. (This is because multiplayer games played from a single cart have to transfer the game data- usually in cut-down form- to the other machines via the link cable. It will certainly try your patience, and is not offered as a feature by all multiplayer games.)
The GBA, through marketing muscle, competent hardware, strong third-party support, and a lack of almost any serious competition, is now the first choice in portable games systems. Although there is definitely room for improvement, at least you should be able to find games to suit nearly all tastes.
March 2003 sees the launch of the Game Boy Advance SP, a redesign of the hardware that addresses some of the problems with it. There is also a Game Boy Player peripheral for the Nintendo Gamecube which allows GBA games to be played on your TV.