I haven't messed around too much with the Replay feature but one thing I can attest to;
When trying to use FRAPS to record during a mission the framerate dropped horrendously (my system is starting to show it's age
). However, when using the Replay function it was a lot smoother although a wee bit jerky when the action heats up.
The Replay feature is finicky too in that you have to be careful where you start it and as mentioned the files are HUGE!! LOL.