I've also gotten through about the first 1/3. Based on this, and the table of contents, it goes into much more depth (both in terms of implementation and non-trivial illustrative examples) into a number of topics that are either only touched upon in SICP, or not discussed at all. These include combinators, generic functions, pattern matching, etc. There's a chapter on propagators, which didn't even exist when the last edition of SICP was written, though I think SICP does discuss related (but simpler) ideas on constraint propagation.