Actually, rcov only tells you about C0 test coverage. This is great as far as it goes, but 100% of the lines of code executed is not the same as testing every branch of the code. Our tests may not be complete.
So we've started using heckle. Heckle does cod mutation to exercise your test coverage. Basically, it changes your code, changes parameters, etc. and looks for your tests to fail. If your tests don't fail, then you're not testing that code path.
Heckle so far has found some areas we're not testing as well as we'd like. This is going to be an adventure!