I had the same question to ask as Jaser two days ago- I am using FCP and have been having nightmares with Apples Compressor- and waisted a lot of working hours trying to do mp4s for clients who couldn t play them properly on PCs.
I just got Compression Master and after a few test am really pleased with the result- and so was the client.
I was also impressed with the speed- I am looking at my Activity Monitor and checking the cpu workload and CM looks really efficient here too.
On the test I did- the file I created for WMP was very satisfactory, and tiny compared to the QT files- using given templates.
The lesson I took out from this little adventure was that Compressor is not adequate for a professional job- one needs more choice to offer clients.