I have the BBS 2, bought it for a 4 wt, and picked up an extra spool for a spare 5 wt line. With the 5 weight line, it holds almost no backing.
This causes a few potential problems: IF you want to fish somewhere, such as the Delaware, where it is not uncommon for a fish to take you into your backing, you have almost none to play with. NOT Good! If you aren't fishing in places similar to that, like me, not too big of a deal.
What causes me problems: The arbor is so small and with barely any backing the fly line needs to be stretched before using or the end result is not good. Very difficult to be on top of your game if you don't!
I deal with it, as it is just my backup, and I rarely use it. I would highly recommend a BBS 3, which helps with both these problems. I really like the battenkill, if you look at previous threads on this site, they come highly recommended.
Edit: Just Got My BBS 2. With a 5wt line, it has 15 feet of backing...lol