Compressing something already compressed is pointless. So like emails or source code compress well, but video, audio or game assets do not at all basically. So doesn't matter for those things whether you have it enabled, and the overall percentage of uncompressed stuff on your drive is probably rather low, and mostly very small files.
I think the CPU overhead should be very small, but I haven't tried it myself. Plus if you actually have something that compresses well, it should actually be faster to access because you're almost certainly not CPU-limited, but rather limited by the read/write speed of the disk.
