I love using Perforce, and have used it both in 'paid' and 'free' situations.

I would not consider it freeware, as if you choose to use it 'for free', you are limited to 2 users, and 2 client views, so unless you are by yourself or on a very small team, you need to pay a fair amount per user (and yearly fee's to upgrade the server to the latest version).

This is just a clarification, because the 2009.1 update page in versiontracker listed it as freeware, while the earlier 2004.x page has an actual price.

It's expensive compared to free (cvs, and now subversion, git, etc...), but cheaper than most of the competition. I've used it for 10 years, and the server has only crashed on me once (p4d), and I haven't lost even an update or got a corrupt file from the crash or multiple forced restarts of the machine hosting it. It's also got clients for a crazy number of platforms, including both a GUI client and a cli tool for Mac OS X, and it's supported within XCode. The initial learning curve for setting up and using the client is pretty high, and it may take a couple of projects for you to figure out the best way to organize the source in Perforce for your specific needs.

As a guidepost, I would rather kill myself than use CVS, I'm not a fan of distributed scm's like git or mercurial, I wouldn't touch Seapine's scm with a ten foot pole after having used TestTrack for years and having it crash with even light (as in 2 users) usage and corrupt it's databases multiple times, and haven't looked at systems that don't have a native Mac client. I may switch to Subversion now that they have improved handling of branches and there is more mature client software available, but there doesn't seem to be anything really compelling with it over Perforce other than it's free (server and basic clients).