Larry Jordan did an extensive comparison of the latest Apple Compressor to Adobe Media Encoder.
He ran the tests twice, as at first he used default settings and didn’t check image quality, but his seconds test are more equivalent, and the results surprised me as I gave up on Compressor a long time ago.
Basically it looks like for Single Pass encoding Compressor is faster, but for Multi-Pass (though it defaults to single pass) or resized compressing Media Encoder is faster, and it looks better at lower file sizes, though Compressor has more ability to customize settings.
The article is certainly worth a read, especially for all those who do a lot of compression.