I've done 1 custom with a bust-up so far, and that was my Gand Jedi. The Zuckuss head/hands fit perfectly on the ROTS Kit Fisto body. I think the Bossk works well also, but I haven't tried it.
I was really excited about making a bunch of custom Mandalorians with the Bust-Ups Boba/Jango Fetts, but they seem to be underscaled (especially when put next to the Series 2 Stormtrooper)
The Han Solo and Luke heads are a little too big, in my opinion.
The Stormtrooper works well (the helmet of the bust-up is slightly larger than the VOTC helmet, so it works well if you're going to hollow out and put a head inside).
I can't wait for the ROTS Clones bust-ups!