I picked up an x-9 medium cage der. Supposed crisper shifts, less chain slap, and some other stuff. Had it for about 4 months (?), but not that much riding (winter). Maybe 15-20 rides or so.
From what I can tell it seems shifts nicely compared to my old x-7 that I bent the crap out of, at least from what I remember. The shorter cage is also nice b/c not as close to the ground.....very slight diff, but who knows how that extra bit might help out.
Big/small gear is a no-no as the der will fold back in on itself, but that much chain cross isn't good anyway. However, i've gotten away with 'almost' the big small combo (by accident of course).
Hope that helps