I ran an evil campaign (currently on hiatus/over) and thankfully, I had some good rps. I agree with the above post about evil being in motives. The PCs were generally for sale to the highest bidder, which meant at time they would do good for the right price. Their was the party NE assassin who was indescriminate in who he would kill, he prefered humans, but was also greedy and wouldn't kill someone powerful unless their was treasure promised or on the person.
When I was running the game, I tried to avoid describing truly evil actions in vile ways. Despite the partys X-rated actions, I would place the game at a PG-13, this is mostly due to the fact that IRL I am a very nice, peaceful guy. Still, it was a lot of fun (and at times a real challenge) to run.
Someone mentioned their DM throwing droves of good guys at them. That was one of my primary encounters at every session. Essentially, the PCs caused WWI by assassinating an important dwarven archduke and attracted the attention of MECHANUS. Their BBGG (Big Bad Good Guy) was a cleric from Mechanus who wanted to arrest them for throwing the cosmos out of whac. Preety much every session, they'd be on the road, and all of a sudden an inevitable or something would teleport in, attempt to dispense justice and they would battle. Evenutally, they attracted the attention of the forces of good as well and got some justicators and paladins and stuff.
The evil campaign was a challenge because I couldn't acurately plot the characters actions to account for story; they were too unpredictable. This was preety fun to run because it was so open ended. The PCs would come to a town, try and formulate a plan to get some money, usually execute it with a few variables, and move on. It would be possible for the PCs to kill anyone, but generally they would only do this if they had a reason (like they liked the ring the NPC was wearing).
running an evil campaign meant that the hooks had to be much more generalized. Instead of rescuing a princess for good, they might do it for money and then (maybe) return the princess. Also, they had a long list of people (evil,good, and otherwise) that they screwed over or pissed off somehow that were preety much were a constant threat so the party traveled with nondetection spells, undetectable alignment, and were constantly looking out for scrys (on average 3 at all times on them). It was a difficult existence for them, but that's what they got for being evil.