Personally, I prefer both the Boss CS-3 Compression Sustainer as well as the compression portion of the Visual Sound V2 V266 Route 66 Overdrive/Compressor pedal, but the MXR Custom Comp isn't bad.
Of course, none of the pedals I've mentioned are as good as the Keeley Compressor--that's the best one I've ever heard so far and the compression in the Route 66 is one of the next few in line...out of the compressors I've heard, of course.