Gonzalo Ayuso's Blog: Database Abstraction Layers in PHP. PDO versus DBAL
In fact DBAL isn't a pure database abstraction layer. It's built over PDO. It's a set of PHP classes we can use that gives us features not available with 'pure' PDO. If we use Doctrine2 we're using DBAL behind the scene, but we don't need to use Doctrine2 to use DBAL. We can use DBAL as a database abstraction layer without any ORM. Obiously this extra PHP layer over our PDO extension needs to pay a fee. I will have a look to this fee in this post.
He includes a few simple benchmarking scripts that compare similar operations done by either, including memory usage and execution time. PDO comes out on top, obviously, because it's an extension versus a set of PHP libraries that need to be included in the application. He does include examples of a few things he likes that DBAL does that PDO doesn't by default - a transactional mode, type conversion, a simpler binding interface and the ability to nest transactions.