7 strings are the BOMB. After taking a year or so to get used to it, I'm not looking back. Think about how many times you've played in standard tuning...you're playing some nice deep chords and then you reach for the "D" chord--BOOM...instant pussy sound. Now, you just slap your thumb on the 3rd fret of that magical extra string and dreams of not playing like a curtain shopper become an instant reality. That being said, here's what you have to watch out for:
1. Block off that floating bridge. Seriously, tuning these with a floater just sucks balls.
2. You're not gonna find too many moveable barre shapes that use all 7 strings.
3. Trying to avoid that B string when you just wanna rock out old school by smacking that E repeatedly isn't as intuitive as you might think.
4. You get to relearn the modes.
5. 7 strings aren't just for nu-metalers anymore. The clean channel chords you can play with these things can sound pretty godlike. Watch out when you're recording them, though...that low string has a tendency to muddy things up if you're not careful.
6. You're bass player really gets pissy when you write a song in B minor, because now, you go as low as he does. Time to buy a 5 string!
7. EVERYBODY that plays my guitar hates it. "The neck's too fat." "I can't even tell where I'm at." "My dick is a lot smaller than yours." Etc. They do take a little getting used to.
