I think I'm halfway done with the main story, but I'm not sure. I've heard it's lengthy, but good. So far the storytelling is leaps and bounds better than the first game, now that half of the plot isn't being told through cryptic trading cards. I haven't gotten to the Crucible yet, but the beta had a better balance of gunplay and subclass abilities than the first, too.
My verdict: If you enjoyed the first Destiny, then D2 is a must buy. If not, I'd rent it first to see if the changes Bungie made were enough for you.