Heavy and the gears arnt as crisp a change as a well tuned derailleur. Noisy would be my biggest beef.
Saying that, the low maintenance sold me on mine so I'm happy enough with it. Many dont like the 5th and 7th gear noise which dont bother me, as its the 8 to 14 over run noise that I detest. Basically the clicking whilst coasting in these gears is noisy, obtrusive and embarrassing (IMHO).
Not having to worry about bent derailleur hangers is great out in the back blocks of Australia and self destructing rear mechs are a thing of the past.
I dislike the gear change which for me involves having to grab the grip shift twice to go from high range to low due to my wrist not having the revolving ability to turn it in one "throw".
I put up with these small niggles due to my perception of it being a a low maintenance and adjustment solution which works fine for me in my Surly Ogre.