It may not help if you have stuff that's running hot, but I think it's a good idea for any of the Unibody MBPs. I installed one a few weeks ago and it's made mine considerably more pleasant to use. Almost no "beachballing", Finder shows all files almost instantly, opening/closing programs has little lag, and it just runs considerably better.
I was talking to an employee at an Apple Store, and when I mentioned that I installed an SSD, he said that's typically the best thing that someone with a Unibody MBP can do. It may not be as fast as the newer Macs with faster SSD-only interfaces, but it can make typical lag times imperceptible. A new one might be a nice shiny toy with a nicer display, but for routine use an SSD can make a Unibody (especially one with SATA III speeds) feel pretty close.