Using Doctrine it is fairly easy to "break out" of the ORM. Doctrine lets you write queries in 4 different ways:
- DQL. Doctrine's own query language that comes with all of the benefits of Doctrine.
- "raw" DQL ("Native queries" in Doctrine2). This is similar to DQL but allows a little more flexibility in commands (e.g. database specific features). In this mode, you'll have to specify a little more about how components are related to one another.
- SQL, using PHP's PDO. You can use a Doctrine_Connection to get a PDO instance which lets you write queries but still have the added safety and ease of use granted by PDO.
- raw SQL. While I'm not sure why you'd want it, I think Doctrine provides this, if not, you could always break out of Doctrine entirely.
If you're using Doctrine inside of Symfony, there are absolutely no features of Symfony that lock you into using Doctrine, even if it's enabled.
One final warning: if you're using some of Doctrine's advanced features (e.g. events or behaviors) these will become difficult to tie in when you do queries outside of DQL.