On Rdavid.net there's a post with the results from some tests run on hos much memory the Zend Framework and Doctrine used both with and without the APC caching.
I have decided to run with Doctrine as my Domain Model in Zend Framework projects. The thing is, if I'm going to commit to this, I need to know that applications I build in the future with the Zend Framework while using Doctrine as an integral part of the Model layer will not take performance hits from things like memory usage. With Doctrine doing a _lot_ of magic, I thought that this would be something that I wanted to see for myself. 4MB Memory to execute a simple Query?!?! Ffffff#$#!!!!
He includes the code for his testing procedure - creating a basic Doctrine object and running a "fetchOne" query and measuring the memory consumption with the memory_get_usage function. His results with the APC caching came out faster by about 60-70 percent.