I used the Creation Audio Labs Redeemer, it's incredibly transparent and doesn't eat batteries (6 months and haven't replaced mine) I've been pretty happy with it.
http://www.creationaudiolabs.com/redeemer/
Their website is a little over the top, they make the thing sound like it cures cancer, which it doesn't, it's just a really nice buffer. The one cool feature is that when the battery dies the guitar still works even if the buffer isn't bypassed, it has some sort of internal bypass system.