I use it on my MBP as it contains sensitive information and it'd be rather bad if it got lost. I don't notice a hit during use, but when shutting down/logging off I get a FileVault progression bar - how long it takes depends on how much data crunching I've been doing and how much it needs to encrypt.
Yeah, it all depends on how many changes to the files have taken place...
Also, realize that it doesn't encrypt the whole drive, only your account directory. If you want full drive encryption, look into something like PGP....