There's no doubt that DB has a great and unusual story and exciting twists that really add to the game. However, I only completed their quests once and even after that I reloaded my save and destroyed them.
I don't have a problem with the assassinations... heck, there's a civil war in Skyrim, lots of freaks, worshipers, necromancers, draugurs, etc... are walking on the ground so "killing" is not something that anyone could or should avoid. Besides, people that are on the assassination list are usually, though not always - bad guys.
The problem I have with DB are their personalities. When you talk to them, every single of them is a psychopath. Unlike Thieves guild, that are bunch of usually cool and badass dudes and girls, DB are freaks. Sure, Astrid and her husband are very unique and interesting, not to mention Cicero, but... it's kind of hard for me to digest them all at once.
I'm on my way again - to finally stick with them or destroy them... sigh!