Well, I just bought the game, and I'll say this: it's not a patch on Pokemon Stadium 2.
The RPG mode is...fine. If a little tiresome. But the whole game just feels a little empty compared to Pokemon Stadium 2. There's a ton of stuff missing, and it most definitely shows if you were a fan of the N64 games.
Visually, it's pretty great if you ask me. The attacks look a lot better, for one, and the new Pokemon look ace (although what's up with the fat Absol?). One thing that sort of clashes though, is the N64-esque graphical style of the R/B/Y/G/S/C monsters, and the almost cel-shaded look of the newer Pokemon (Salamence and Swalot are prime examples of this). Hydro Pump also looks like it's come straight out of The Wind Waker.
Altogether, I'd give it a 6/10. Hopefully the next Stadium/Colosseum title will include all the extras from the two Stadium games, and not cut things out just to big up a gimmicky 3D adventure mode.