How much of your code do you unit test? Do you try to get 100% coverage of everything or do you skip some classes?
We've made a lightweight ORM layer which we have written plenty of tests for. On top of that we use Code Contracts to specify what each method in every interface should accept and return.
Classes to test or not to test
We create repository classes to fetch items or collections. These classes are basically SQL queries and calls to the ORM layer. We also have entity mapping classes similar to those in fluent nhibernate.
Why I ask
The reason I'm asking is that I don't really see the point in writing unit tests for these classes. Well, there is a point if we could use a test database to see that no exceptions are thrown. Our test databases are a bit messy and are never reseted, hence it's impossible to validate any data (and that is out of my control). We could use a fake/mock ORM layer to try the repositories, but I do not see the point in it.