I've never really used a compression pedal, do they work good for metal?
Zakk Wylde uses one. I like them, butt it's up to personal taste.
Heavy distortion adds a lot of compression, so there's no point in using a compressor if you play high-gain stuff. Unless you want one for your cleans.
it would take away from dynamics a bit. If you wanted a sustainer, use a distortion or fuzz like a BMP or something.