How can you tell which actually work and which ones do nothing at all? There is not a lot out there to test with
So at the moment they should have a 100% detection rate. Hell, they should be able to use heuristics to detect any software that makes use of known exploits. But that's my concern with the Mac at the moment - it's getting actively targetted with malware, but there's no idea which software properly protects. At least with Windows you know that MSE is free and relatively non-intrusive, so it isn't a case where the cure is worse than the disease, and it does actually catch malware, or alternatively if you buy WebRoot Secure Anywhere and install it on a clean system you're close to 100% guaranteed you won't get any new infections. The devil you know is better than the devil you don't.
Right now I would just pick a free Mac AV program that uses the least resources possible, run FireFox with NoScript and Web of Trust and make damn sure I know what I'm downloading.