I'm interested to see how many people are using this, and some details on their setup, like what flavor of linux they're using, how many cameras, what size hard drive, etc...
I think it is great. I have played with it off and on for a few years.
Right now, I am running a test on a Celeron 800 system with 2 gigs of ram and an 80 gig hd running ubuntu. I have two IP cams feeding it over the network (panasonic bl-c10a's).
The cams support ptz and are controllable within zoneminder. I only get about 3 fps off each cam on that system, tho.
I am not a linux guy, so everytime I experiment with it, it is a major pain to get actually working, but once it is, it is great! When I get some disposable income, I hope to get a quick dual core box to put combine both zoneminder and asterisk on...with a real capture card and some additional cams.
One of the nice features is streaming in firefox doesn't require any kind of client app. On IE, you'll use a free plugin called cambozola.
Personally, I don't know of any other software comparable to zoneminder...