Putting on a dark coat of paint on the same old game
Pokemon Colosseum's biggest perk is its dark story (in terms of cute little monsters making each other 'faint' its rather dark). You are a trainer who used to be a part of an evil organization and now you are just trying to get by as a Pokemon trainer. However, your former employer doesn't take to kind to deserters and you eventually start to have deadly encounters with them. The story is a welcome change to people who have always wanted something different from pokemon.
The gameplay mechanics are tweaked to help Pokemon maintain some freshness while still being familiar to those who haven't played since the Game Boy games. Battles are now 2-on-2 and there even more items to equip to your Pocket Monsters. During special encounters, you run into Pokemon who have "black hearts" or something like that which makes them 'evil Pokemon'. Once you beat the hell out of them-- I mean 'free' them-- they become yours. Although I personally would have preferred the old method of just catching wild Pokemon, this works just fine (and it doesn't hurt that the controls are very straight forward). And for people who have a Game Boy Advance and some link cables you can upload data from Ruby, Sapphire, Emerald, FireRed and LeafGreen.
While it puts some interesting new ideas in the PokeMon universe, the ideas quickly end up becoming window dressing and still don't change the fact this is the same PokeMon you have been playing for years. You still have to save at PokeMon Centers, buy items from PokeMarts (haven't these people heard of Discount Cards?), and collect Gym Badges.
On top of that, the graphics don't do the game any justice. While the frame rate is good, the color scheme is just as dull as any other generic future games. The Pokemon creatures have no shading, texturing, and battle animations as limited as they were in the Nintendo 64.
The Bottom Line
Overall, if you are still a rabid fan of Pokemon, this is a game you could easily play and you like it as much as the other games. If you arn't much of a Pokemon fan or were a Pokemon fan who has gone on to other media, this game won't bring you back into the fold.