I played and loved the first 2, just started Brotherhood recently and I'm really liking it. Part of the appeal is that I've spent a lot of time studying renaissance Europe so the setting is a lot of fun for me. It's odd seeing Machiavelli dealing with mercenaries though, he's on the record as hating them.
I wish there was more play time as Desmond, I'm actually more intrigued by his story than Ezio's. I'm glad they're bringing in a new lead character, I feel like they didn't do enough with Altair and they've done too much with Ezio. I haven't played Revelations yet, but I will once I beat brotherhood.