Rajarshi Guha, together with Dazhi Jiao (Uni. Indiana), have put together a test suite for SMARTS matchers, and run the test suite against the CDK, OpenBabel and OEChem. Greg contributed results for RDKit. The overall performance is described on Rajarshi's website.
Here's the current summary of the results for the 158 test cases, but check Rajarshi's page for a more up-to-date picture:
Image: Everybody stand back by Chris Radcliff (CC BY-SA 2.0)