I think it had the most potential of all the Guilds, but it just seemed sort of .... meh to me. The one mission in Skingrad to me is the perfect example of what the DB could have been, and what it became. The devs had a chance to build a totally Agatha Christie And Then There Were None type story full of intrigue and fear. Instead it was filled with a bunch of one hit kill NPC's that was so laughably easy to complete that it lacked any challenge at all.
The only things I liked about the DB was the Friday the 13th reference, and the chance to kill that punk-assed Dunmer from the beginning of the game. Other than that, I really think the DB is overrated.
edit - I agree with Savlian.
I thought the skingrad mission/quest whatever, was completely out of character for the DB quest line and one of the more non satisfying quests. Talk about killing weak oponents, you have them all in the same house, not that hard to get them separated and then whack them. Agathie Christie is all well an good, I enjoyed her books, but to me the DB wasn't necessarily a cloak and dagger type guild, but more of a "hitman type" guild. If the money's right, then people die.
The Thieves guild is cloak and dagger, minus the dager...