Signature Abilities in their current, incomplete state are wildly out of balance with the rest of the game. I'm not complaining that they're clumsy, clunky or overpowered; they are, but in a way that usually adds to the narrative and the role-playing of a session.
No, I mean they're broken in how they are available to the players.
My main issue is a result of two problems. First is that to provide a complete catalog for the group, you'd need to have the cash for 6 books at 30 dollars each, and double or triple that if you're including the multiple game lines. That's 180.00, 360.00, or 540.00 dollars, respectively. Not a small amount of money. Second, even if you are buying every career splat book that's come out, you'd still be limited by the fact that they haven't all been made yet.
This means that some of your players and my players don't have access to what's touted as the capstone of their abilities.
What I've taken to doing is allowing any PC to attach one Signature Ability to any tree, regardless of career, as long as the tree has thematic ties to the ability. Each PC may take only one, and once an ability is taken, it is unavaliable to other PCs (I think it makes each ability a little more special and Signature).
What solutions have you used for your games? If you simply use the Signature Ability rules as written, how have they worked at your table?