I caught my first bug (regression) because of WatiN and NUnit just now. Something I wasn't able to catch during my previous manual tests. [more]
Two items that I run into but got over with:
1. AJAX (UpdatePanel). I show/hide controls with AJAX and previously finding elements by id had some issues if it's hidden and AJAX has still not finished showing the said control. Resolution: WaitUntilExists (and the likes)
Finished my first set of tests. Insert, Update, Load, Delete pages for one entity and doing fine so far. Hopefully until the end. So I would suggest you try out WatiN yourself.