I understand that unit testing should cover most of scenarios if an application is nicely designed with some pro-unit testing framework, namely MVC pattern. In .Net world, those unit testing friendly frameworks include Asp.Net MVC, and Castle Project. But a lot of times, an application has already been designed and put into the production, you don't have time, money, and energy to re-write those anti-unit testing applications based on unit-testing friendly framework. Mostly importantly, business users don't understand why you want to rewrite an application just because you want to be able to unit testing it, they just need a workable application, and don't care too much about the back end unit testing which is an essential part of the whole project.
I am working on an application which is heavily depend on CSLA.Net framework, which is a very impressive framework. However, the framework is not very TDD friendly, and check out some discussions in this post. I started to look for some other alternatives which can automate the testing process, and the visual studio team system had an edition for software testers starting from Visual studio 2005. But honestly, I won't recommend using it unless you have the VS2008 version. The biggest limitation of VS2005 test version is that it doesn't support Ajax testing natively, and you have to rely on fiddler tool to record the testing. VS 2005 test version cannot detect dynamic parameter very well, and I have big trouble to get those viewstate related variables work. VS 2008 version definitely did a lot of improvements in those areas, although it's not free of bugs either. Check out an excellent post on web test in visual studio 2008 here.
A couple of roadblocks I have in working on the automating tests are:
1. You have to turn page validation off and view state check off. You probably want to turn it back in production environment.
2. You may need to hard code a machine key if you want to port the test to different sites. In our cases, we recorded the tests on a test website, but we also want to run it on stage site too.
3. Some post parameters are promoted as dynamic parameters incorrectly, watch out those if you need manually correct those.
4. Check out this web test plug in if you want to use some .Net functions in your test.
Sometimes, you want to create set up some specific pre-testing condition, and tear down those values added to your database in your testing, you can create a plug in to do this work.
In our projects, in PreWebTest, we create an object in the database which we need run our test on, and in PostWebTest, we delete this object. It works very well for our situation. We run those tests every day , and those green pass icons definitely make our life much easier.