7.5 offers full-time scanning of what's coming into and going out of your Mac. On my 667 PB, it took way too much of the processor and RAM. I also couldn't figure out a way to get the dang thing to stop loading on startup. So, off it went!
I've now got 7.2 on my PB, but it doesn't load at startup, it's there only for if I think there's a problem.
I've always been a supporter of having some kind of virus protection on your computer, even a Mac. I've changed my mind. I'm going commando!
Don't bother.