I did this over a year ago and am very happy. I use TrueCrypt. I have not noticed any slow downs (even for gaming)... That said, you should probably not do this

I did this over a year ago and am very happy. I use TrueCrypt. I have not noticed any slow downs (even for gaming)... That said, you should probably not do this if your drives are failing (or you tend to suffer a lot of disk failures).