I've had my Fuji Cross since new in spring 2003. It came with 9-speed 105 drivetrain. Current crankset is FSA Gossamer with 46/34 rings. Several years back I switched to a mountain bike rear derailleur so I could run a 11-28 or 11-32 cassette. I also have a no-longer-used 2006 K2 road bike that has full 105 plus Ultegra rear derailleur. This handles up to a 28 big cog.
The bike sees a mix of commuting (on 700x28 tires), gravel/dirt road & trail rides (did 130 miles on a single June day on the C&O Canal Towpath), some just for the fun of it single-track in the woods, and a couple of CX races. I've had a really good spring & summer on the road bike and hope to take advantage of those miles by training for and racing in 5 to 7 CX races this fall.
The question I am wondering is whether it would be worth the effort to move the 10-speed group to the Fuji. I might miss the 32 cog on some of the single-track stuff but not anyplace else. The 10-speed group shifts better as currently set up. I can do all the work myself so the only extra costs might be cables & bar tape (probably could use new cables for the Fuji regardless).