Transition Initialy is weird. I had to change my left hand to a classical guitar position, which in the end help my playing tremendously on a 6 as well, no more thumb on top of neck, now always in the middle
Then it took about 3/6 months to start experimenting, and now about 18months later I am completely comfortable on one. 6's feel a bit limiting sometimes
, now it allows a wide range of playing (loads of positions) and is just more fun creativity wise. Also, 7string pickups sound tighter than a 6string downtuned to B (if you get good pups)
Oh, and get one with good pickups and a nice neck (along with a good stable trem system, a decent floyd not some shit budget copy, the ibanez lo-pro edge systems are fantastic), it'll make life easier and you won't end up cursing 7's for shit playability, so please! stay away from all the low end crap that a lot of compnies flooded the market with to entice 14yr old korn wannabies who just wanted a 7 because it's "heavier"
Gauge wise I prefer .009's->.056 on mine for BEADGBE tuning, gives enough slinkiness for soloing, and a tighter feel to the bottom end for chords (stops the low B feeling too floppy) , basically get a light top heavy bottom (9->46) and slap a 56/58 on it for the B, this will also allow you to get really low action as well which is cool (especially on the treble strings) Speed!!!!!!!