I've not noticed a great difference tbh - slightly longer battery life but with an Atom processor, I don't think the disc was the weakest link before.
My MBP has a 200Gb 7200rpm HD which gets tooooooo hot. I would love to rip it out and stick an SSD in but there is so much stuff I could get for the same money....... another 24" display for the mac pro, CS5 (soon), a fish eye lens...... etc etc..... So I think I will end up sticking a 500Gb 7200 drive in instead.