We're using Symfony as well, and it's been quite wonderful. Fitting into the framework took some time, but its abstraction allowed us to move from mysql to posgresql for testing, and then back to mysql without much thought at all. Also, optimizing queries for a specific database is fairly simple.

Propel has been a bit of a kludge at times, but it does a decent job. We're experimenting with 1.3 and Doctrine to evaluate our migration path however.

Anything that's hit you as particularly inefficient?