I like the center diff mostly because it limits front/rear diff and center axle maintenance. With the 2-speed, there isn't any give in the driveline and the diffs XTM has in that thing leave something to be desired as far as strength is concerned.
I've only run the center diff for about 3/4 of a gallon. I lost some punch coming out of the hole and I obviously lost some top speed, but it's still a good basher. It seems to turn and brake better now as well. Not really brake better, but with the center diff, it allows you to control your front/rear bias.
After spending about $40 in plastic spur gears and another $30 for a new rear diff, I bought the center diff.
I do kind of miss the punch off the line, but my axial 28 makes up for the loss. I think I could get away with gearing a tooth or two higher and still have as much snap off the line as I did with my OS 21 RG and the 2-speed.