UNFUN
PCs as a group of amoral mercenaries.
Oh, I don't know. In the "evil campaign" I played in for about three years, we really ended up playing more of an "unaligned campaign". We stuck together (for the most part), and the main differences between our party and the standard party of good guys was that we required a real motivation to do things, we weren't afraid to use "unsavoury" means to gain power and achieve our objectives - we had a player necromancer for a while, my character was a thrall to a devil.
That said, we weren't really all
that evil. We didn't set out to ruin people's lives unnecessarily, even if our main goals usually turned out to be conquest and tyranny.
I think the thing to avoid in an evil campaign is "obnoxious evil", like you have the choice to be in most computer RPGs. Where the character is just nasty to everyone around him for no good reason at all. For instance, in Knights of the Old Republic, there's a point where you're dealing with a tribe of primitives. Their village elder knows of a promised land he's trying to get the tribe to where they can all live together in peace and harmony. You follow the clues, and come up with a map to the promised land. The local shopkeeper is fleecing the villagers for supplies, and offers you 100 credits (a pitiful sum) to burn the map. The evil choice is to do it. This is "obnoxious evil". You're just being horrible for no reason at all. I'd far rather laugh in his face, show the map to the village elder, then tell everyone I knew that the shopkeeper asked me to burn the map for his own gain. Then I'd watch him reap what he sowed. Far better, and less obnoxious.
So, I'd say an amoral mercenary campaign can be fun, cause I've played in one. It was a very liberating way to play, and probably the best campaign I've been in so far.