Ok, I'll try it like this

Pros
Few extra bottom-end notes
more chord formations to work with
A different versatility in accessing already available notes (low E notes are on the low B only higher up and easier to reach if you're playing around, say F on the A string)

Cons
Wider neck

My personal experiences and evaluation
Ok, I strained myself to think of more cons than that, but that's all I was really able to come up with, so anyone else feel free to contribute. That being said, I'm 50/50 on the wider neck problem as I've played on a 7-string a few times and really didn't have any problem with adjusting to the wider neck. Truth be known, the only type if instrument with a wider neck that really bugged me was a 6-string bass that was just built with a neck cumbersomely wide for my reach. Other than that, to me it's on the same level as learning to play sitting down as well as standing up.