I will hear no bad things about Great Wolf Sif. It's a well designed, well implemented boss fight! Now, Flamelurker on Demon's, that's another thing entirely and it can go choke.

Personally, I've never liked boss battles. I don't mind fighting a stronger enemy, it's just that boss battles are often about beating the boss in a very specific way that destroys creative problem solving.

So, as long as I can choose how to beat the boss and I get some feedback that what I'm doing is working, then they're fine. Otherwise they're just annoying.