I've had it since yesterday.
On the RPG side, it's really neat! Walking around in 3-D with Pok?mon is cool. I saw a tiny little Castform circling a pool, for instance. All the battles in RPG mode are double battles, and you start with Espeon and Umbreon. Snagging Pok?mon is a bit harder than catching them, since the trainer can heal them with potions/have their other one attack etc.
The Colosseum mode is cheap...this one girl had Rayquaza and Latias, another had Kyogre. It's much much more difficult than Stadium, which is good. It's not nearly about connectitivty as Stadium was. You don't really get anything for your GBA game, and the non-RPG mode feels kind of shallow. You have to register your in-game team to the memory card. This is to 'save batteries on your GBA'. So basically you register your team(s) and use them in battles for Pok?Coupons, which can be redeemed on either the GBA or in RPG mode.
I think I like RPG mode better at the moment, it's quite fun battling in 3-D yet gaining exp etc.