havent tried watchers keep yet it seems pretty hard...is it hard?
and is it fun?

It is hard, and occasionally dull, but the rewards-bragging rights (you'll know what i mean when you've done it :D ) and the brilliant, upgradeable weapons that can be found are well worth it.

Fun? Sure. Different too. Each level has it's own distinct feel and the whole keep is only loosely tied in with the main plot so it may come as a much welcomed change to the whole Bhaalspawn thing.
Spoiler
P
O
I
L
E
R

And when you get to the end, don't be a sissy. Kill the big D. It's well worth it;)

well It adds more game play to any already good game. It is a nice break from the plot. And yes the weapons are worth it.

I would say go and do it.