When released, Sonic Advance marked the first time the blue hedgehog appeared on a Nintendo console. It was also significant for being the first solid platformer on the GBA that wasn't a SNES port. More then anything else though, the game returns Sonic to its roots. The plot is non-existent, the levels go from left-to-right and the newest character present is Amy.
Right from the bat there's 4 characters to choose from: Sonic, Tails, Knuckles and Amy. Each have things that make them unique, for example Sonic can grid down rails, whilst Knuckles can climb walls to find new areas. Generally Amy is the weakest character, but it's not a huge difference. The variety is a plus, and whatever character you choose you go through the same levels.
The levels are also undeniably Sonic. Jumps, spikes and loop-the-loops are all present, and there is a reasonable (but not excessive) sense of speed at appropriate points. The game runs very fluidly and the characters are responsive. The game handle the speed extremely well, and the graphics are one of the high points of the game.
Level design is generally very good, with many levels having an exploration element if you don't blast through it asap. However, some of the later levels are quite unforgiving, with a single mistimed jumped leading to unavoidable death. This can be frustrating, but it doesn't take long to learn which areas require extra attention.
If you do choose to explore a bit, you can find bonus rings, extra lives and even an item that attracts rings towards you. If you're lucky you'll find a special spring, which takes you to the bonus level, with the ability to gain a chaos emerald. Unfortunately, the bonus levels aren't the best, due to the attempt at 3D making it hard to judge distance and position to a great deal of accuracy. Never-the-less, finding the special springs and collection the emeralds does add value to the game.
The soundtrack is pretty decent, with every zone getting different theme music. Many of the sound effects (for jumping, spinning etc.) are distinctly familiar from previous sonic titles, which is a nice touch that adds a sense of nostalgia.
Other then the main game, there's a time trial, a Vs. mode (which is reasonable, although nothing amazing) and the 'tiny chao garden', which is the equivalent of a tamagotchi. You can trade chao's onto the Cube Sonic games if you have the relevant games and cables, but it's all a bit of a pointless gimmick. The garden itself has a couple of mini-games (a version of rock-paper-scissors and a memory card game), which aren't too terrible if you want a quick break from platform hopping.
The bulk of the value is in the main game, however. The game also automatically saves the highest level you've reached, so you can continue from any level beforehand. With 14 levels the main game isn't the longest, but the levels are a decent size and include 7 boss battles, so it's not as bad as it sounds. Replaying levels and finding new routes is also a surprisingly rewarding experience.
Overall, Sonic Advance is the best Sonic game I've ever played (although I haven't played any of the ones released in the last few years). It's a must-buy for Sonic fans, and also recommended for any new to the series...although of course it is about 7 years old now.