i only put an SSD in my macbook yesterday, its battery life gained over an hour (estimated by osx) and is silent! i can only hear the CPU fan if i put it to my ear.
Id recommend an SSD over a mechanical drive any day.
Now obviously size is a major issue, unless your rich an SSD is going to be around 1/10th the size of your current drive.
It doesnt effect me as i have a NAS system and a server to keep data flowing so i dont have to have large amounts on my mac.
My gaming rig is almost full with only Bf3, skyrim, red orchestra 2 and saints row the 3rd installed. but playing games through a network wouldnt be the best of ideas, so i guess ill have to stick in another SSD or 2 for now