Yes, absolutely worth it. You won't see things like Wicked, Spamalot, or Jersey Boys--think more of the kind of shows you'd see available at TKTS (which is run by TDF)--but who cares? They usually have some fantastic Broadway shows each month that I've been wanting to see but wouldn't have otherwise spent full price on (also a lot of great off-Broadway fare that I wouldn't have even heard of otherwise). It can be a great way to catch shows before they close--if the shows aren't selling enough and are on their way out, they'll sometimes make those unsold seats available through TDF for a fraction of the cost. I saw Dirty Rotten Scoundrels about a week before it closed and paid $33 for a great $111.25 Front Mezz seat...couldn't have been more satisfied. So definitely go for it.
"If there is going to be a restoration fee, there should also be a Renaissance fee, a Middle Ages fee and a Dark Ages fee. Someone must have men in the back room making up names, euphemisms for profit."
(Emanuel Azenberg)