You may argue that this is not unit testing but functional/acceptance testing (an I should use Selenium for that). But you need functional testing too and Simpletest is a good tool for the job, as long as no JavaScript is involved.
I also prefer the Simpletest implementation of mocking to that of PHPUnit.

Don’t get me wrong, it’s a well developed framework but it lacks a lot of features found in PHPUnit, and the PHPUnit community is more stable and growing at a pace quicker than that of Simple Test.

You’ll find that Simple Test as in regards to it’s loyal user base appear to be those from Sitepoint’s PAD forum for the most part, as from my experience most people not assoc. with that forum use PHPUnit by default.

Few people outside the regular posters of that forum haven’t even heard of Simple Test never mind used it… That alone speaks volumes in my opinion.

btw, building after each commit can be annoying: say you forgot to commit one file - the build blows up. so you need another commit and another build.
you can avoid that by setting the idle time after a commit to 5 minutes (or whatever) and CC will collect all changes within those 5 minutes. should be enough time to check the commit logs