In our project, we have 20 simple web controls, 8 composite web controls including Ajaxable Custom Grid. And all modules are developed dependent to these controls. 

And now I afraid to change a control, because it effects modules more then I think.

It's too late I know but I need to write some test cases for my controls.

I searched web and find WatiN. It look good, but I need to know is it effective ?

And also I want to hear your experiences on WatiN or other tools. If you have different solution / technic to test your web applications/core custom controls, I want to learn details, problems you faced.

Thanks for all the replies !