Your tests should be picking the majority of your algorithmic and functional errors, so I'd have to assume you're talking about the state of the code before thorough testing.
What's it like after that testing? How's the long term maintainability? I'm much more interested in possible problems - and it could go either way - in the long term than before I've finished writing it.
I tend not to write oodles of test cases up front in favour of simple scenario based tests applied later on. I add pre/post/invariant-condition checks in the code as I write it. It is tested incrementally by hand.
The condition checks prevent the what if's and tell you why something broke. The scenario tests ensure that it does what is asked of it.
I rarely get bugs raised against my code (7 this year out of about 112,000 lines of c# written). Not bad!
What's it like after that testing? How's the long term maintainability? I'm much more interested in possible problems - and it could go either way - in the long term than before I've finished writing it.