I've run them against a level 7 party with a Batman wizard, a C and Dzilla, and a rogue/swordsage, and was pleased with the results. I can't remember the exact way it went, just that their ability to see through the usual tricks (invisibility, illusions), great saves (thanks to outsider hit dice), flight, oodles of attacks with a good bonus, and multitude of fort saves made them pretty fierce. They weren't party wipers because PCs are resourceful and have access to a lot of equipment, but damn they were fun. And creepy as hell. I really creeped the PCs out with them and their handiwork.