Unless your data is worthy of encryption, then I would not turn it on.
The risk of data loss is far greater with filevault.
When you empty the trash, you do not gain that space back until you restart, which in itself can become a lengthy business as filevault has to resize the filevault image file, which can take an age. During this time you cannot use your mac.
in my opinion, it is more trouble than it's worth.
If you have specific data to encrypt you are better off to use disk util to make a dmg and storing it that way, imo.
I had it turned on for about three or four months when I first got Panther...I wound up turning it off. At that time my laptop generally stayed at the house, so I wasn't too worried about it being stolen or lost. Recently I've started taking it with me just about everywhere, but I've yet to turn file vault back on. I'm not sure if it's been improved under Tiger or not...maybe someone else can offer an insight into that...but I'm not sure I want to take the chances of it being slow like before.
I guess the first question you should ask yourself is do you actually see a need to turn it on? Another would be is this going on a laptop or on a desktop system? If it's a desktop, I'm not sure I'd enable it.
You could always enable it and see for yourself how it works out on your machine...I'd suggest backing up beforehand just to be safe...and if your home folder is rather large be prepared for it to take a while. The more files in there, the longer it'll take initially.