So first, why wouldn't you unit test?

Let's first assume it's a waste of time because you already have 'testers' who will be testing it for you. So you write some code, assume it's correct, pass it over the wall to a tester and begin working the next item on your list. The next day, after you're deep into the work of this second item the tester finds a defect and writes it up for you and you see it in your email. You can either interrupt the work you're currently doing and go address that bug or you can continue working on the item you're on now until it's ready to be thrown over the wall. Either way you do it, you have to re-engage your brain back to the old tasks of that other feature to verify it's correct.

here's some other reasons it sucks (With a VERY nice lively conversation in the comments)

http://www.wilshipley.com/blog/2005/09/unit-testing-is-teh-suck-urr.html

Finally, here is a good article on why to unit test.

http://www.agitar.com/solutions/why_unit_testing.html

I've personally felt more confident in my code once i've written good unit tests. (Right-BICEP - boundaries, inverses/negatives, cross-check, errors,performance) http://www.cs.virginia.edu/~horton/cs494/s05/slides/UnitTesting-lecture-s05.ppt#313,24,Right-BICEP

I also feel way more confident in the designs I come up with when i do TDD. I actually use things like interfaces to decouple my architecture and think of unique ways to get things into a test harness (which makes it easier to be a client for any of my classes i make when i TDD)

Your unit tests are only as good as you are at writing them... so if you do not know how to write good tests you will ultimately get no benefit of writing unit tests.