One of the symfony contributors has posted a comparison of Propel, Doctrine and the Propel integration plugin for symfony. The article focuses mainly on the syntactical differences between the three.
I've been looking at PHP ORM libraries recently, and have spent quite a bit of time with Doctrine in particular. Both libraries have some clever solutions to some tricky problems, I especially like Doctrine's ingenious way of implementing Rails-like acts_as functionality.
I did look at Propel first, since I believe it's been around longer; but was put off by its syntax and focus on auto-generation of classes (which I have an irrational dislike for). Doctrine is a little more intuitive, and the query syntax is a little closer to the Zend_Db_Select/Statement syntax which I'm a big fan of.
Still, it's sad that PHP frameworks have to rely on external libraries for ORM funcionality, as this seems to add an unnecessary layer of overhead for something so fundamental to web application development. The experience has given me a new found respect for Rails' ActiveRecord implementation, and I hope that this is an area that will improve in ZF in the future. Zend have hinted that they'd like to do a proper ActiveRecord implementation post PHP 5.3, it would also be nice to see some components to aid integration with the existing libraries (Zend_Doctrine / Zend_Propel), although these would be fairly mammoth tasks.