Mos of them are fairly beneficial along with boosted stats to make the pokemon actually very usable. --- He who fights and runs away, lives to not have the f****** moon fall on him as an alternative - Tacitus
It sounds like an interesting thing to do if you've become really bored with the main game, but that's all it will ever be... a little side diversion. I'd also be very hesitant to play this because it would taint what I know about Black and White for the metagame.
Nah it wouldn't. I thought that too at first, but you won't get your vanilla B/W knowledge mixed up with BB/VW.
One of the greatest improvements is the ability to evolve pokemon in other ways (so you're not stuck with trade evolution only for things like Gengar) and base happiness being greatly increased on pokemon that evolve by happiness like Golbat.
The difficulty increase is also nice. Burgh will most likely sweep you the first time you battle him, something that was unthinkable in the original versions.
Hopefully a ColdGrey version will be released not short after Grey. I really can't stand the idea of playing the base versions for anything but competitive battling now.