I've skied them for two years now annnnddd here's my opinion:
Powder: fucking sick. obviously. playful and big and float and just do the job like none other when you're playing around. BUT if you're planning on charging big lines, going fast, and hitting bigger cliffs at high speeds and such, they're not number one in my books. They truly are a soft soft ski and so they kind of noodle out on you sometimes, if that makes sense. Not super stable. It all depends what kind of powder skiing you're going to be doing.
On groomers though, they totally hold their own. If you get on a nice slope and start rolling your ankles from side to side a bit, you can rail some mean turns on them.
Park: pretty obvious they aren't the best, but that's clearly due to their weight and size. If you're getting a pair of Hellbents though, I would hope you don't base your decision on how well they ski in park....derp. NOT TO SAY TAHT THEY CAN'T FLOAT CORK THREES OR FIVES OFF BIG JUMPS LIKE COMPLETE BOSSES though. Doing a cork five on a big jump under the ski lift with hellbents ups your park cred like a dungeon quest.
So yeah, there, that's my opinion. And this is from a guy who skis Whistler/Blackcomb all year.