I've paid for Prey. I've not tried Find My Mac yet but if it's like on the iPod (no gps) then it's pretty well useless if it's stolen, as the wifi location really means "probably somewhere on this city block". Prey provides that, as well as a scan of the local network it's attached to, A report of what programs are running and modified files, screenshots of the desktop, pictures from the camera (hopefully of whoever took your mac), lets you clear all passwords, hides your emails, and clear the browser cache, it lets you lock it from being used without a password, lets you send popup messages to it, and play a sound at full volume. Besides all that it also has a handy info pane that has your mac's hardware specs including the serial number, which you'll need to file a police report.
so, to sum it all up, yea it's worth it, but keep in mind that anyone with a brain will format and reinstall before connecting to any network with internet access, and thats why I also run DeepFreeze by faronics. No combination of software will make your mac completely safe, but Prey is a pretty good start.