Given that "enjoyable" is highly subjective, you are going to need to be a bit more specific than that.
I don't enjoy the inconsistent behavior with when DN gets to use annihilate or not. Sometimes its got a CD of 1 remaining, and after his first basic attack it goes down to zero and he annihilates one toon. Usually the CD doesn't change though until after he has taken both moves, leaving annihilate for the following set of actions he gets. (And no I'm not talking about him using force drain for his first action - that while annoying, is at least predictable).
I do enjoy fighting against Sion and Traya though, so... what do you mean by enjoyable?