After playing Resistance many times, my friends and me played Avalon for the first (and second and third and fourth) time last night. The added character roles really add a lot more suspense/confusion to the game. In Resistance (especially with plot cards) it would become somewhat obvious who was a spy and who wasn't pretty fast, whereas we never really ran into that in Avalon. Love it.
Yup, Avalon is fantastic and a big improvement over Resistance.
So far, I've tried everything except for Mordred and Oberon.
After having played about 35 games and getting a good feel for the balance, this is how I usually set up my games:
5 players - Merlin, Assassin
6 players - Merlin, Assassin, Percival
7 players - Merlin, Assassin (optional: Percival and Morgana)
8 players - Merlin, Assassin, Percival
Not sure about 9 or 10 players yet...I've only played those two playercounts early on so I'm not sure how the balance works out. 6 and 8 players, I feel that the Loyals need a bit of help, so I like to add in Percival without Morgana. I'd only ever consider adding Lady of the Lake with 9 players or more, and even then it's not necessary...add her in if you're getting bored and want to spice up the game.
One thing that's really nice about Avalon is that it's really easy to keep the game balanced. If one side is winning more often than not, then just add in a role to help the losing team.