I used Shai Halevi's conference software for STOC. I must admit, I was swayed by Shai's willingness to host the software and help out in a pinch -- I didn't want to deal with Harvard's IT staff being off-hours should a disaster happen. And while no disasters happened, Shai was a great help whenever anything did come up. I (and the rest of the community) owe him thanks for volunteering his time specifically for STOC, as well as more generally for creating the software. I really can't imagine running a PC meeting without such software.
Shai's software seems very similar in spirit to HotCRP, which I've been using recently as a PC member for some other conferences. I like them both. I'd have to say HotCRP seems a bit more polished, like it has gone through some more iterations, and has some more features than Shai's software. On the other hand, if you like control, you can apparently readily modify Shai's code (and the resulting conference database) directly to get what you want. I prefer them both to my experiences with EasyChair. If you're going to chair a conference, you should definitely check out Shai's page.
It might be worthwhile for SIGACT (or some other appropriate group) to look into having a common hosting platform and support for this sort of stuff. (Someone else I knew was using HotCRP for her conference, and when I asked how she set it up, she said that USENIX, the sponsor organization, hosted the conference software for them...) A standard default configuration maintained centrally would avoid having to "re-invent the wheel" each time someone takes over the conference. Or perhaps in the end we'll just leave it to EasyChair, which offers a perhaps less appealing standard default, but at a good price (free, currently).