One of these days I'll have to make it to Sugarbush when conditions actually make skiing Rumble worthwhile... I've skied the various runs off of the Castlerock lift except for Liftline and Rumble. I get the idea that those two trails aren't open very often and are in "good shape" even less often.
All the trails off of the Castlerock lift are really fun, assuming you're a halfway decent bump skier and don't mind a few rocks. Definitely worth skiing those trails if you meet the "qualifications". I've heard they do periodically groom out both "Middle Earth" and "Castle Rock" trails, but whenever I've skied them, the moguls have been pretty sizeable. Maybe it's a "once or twice a year" type grooming, or maybe they knock the bumps down if they've gotten too big / icy.
Weirdest part of skiing the Castlerock area (for me) is just getting on the lift. The chairs are spaced VERY far apart, on the order of about a 30 second wait between chairs. So you get to the loading line and you wait. And you wait. And you wait. And you start wondering if the lift has stopped. And about when you've lost yourself in LaLa Land the chair comes along and smacks you in the back of the knees and you plop down and embark on the long slow creaky ride to the top.