There's some interesting notes, including another bash at the quality of Apple's H.264 compressor (it seems they really need to do something about that):
In terms of [H.264] quality, codec vendors MainConcept (Adobe Media Encoder, Sorenson Squeeze, Rhozet Carbon Coder) and Dicas (Telestream Episode) have been doing an interesting dance for the past couple of years. Specifically, first, MainConcept would pull ahead, then Dicas would catch up, then MainConcept would pull ahead again. In June 2009, the codecs were about even; by the end of the year, MainConcept was ahead again.The comparison includes Apple's Compressor, Microsoft Expression Encoder, Rhozet Carbon Coder, Sorenson Squeeze, Telestream Episode and Adobe Media Encoder.
That said, while it’s noticeable in side-by-side comparisons, the difference isn’t commercially relevant because viewers rarely see side-by-side comparisons. On the other hand, Apple’s codec is a clear step behind and shouldn’t be used unless you have bandwidth to burn and don’t need the optimum data rate/quality mix. The big surprise this year was the quality of the H.264 encoding offered by Microsoft Expression Encoder, which is almost indistinguishable from MainConcept; it’s a pretty incredible performance for a rookie codec.
Streaming Media: Where The Rubber Meets The Road: 2010 Encoder Comparison