I purchased the app a few weeks ago (just before the initial public release) and have been playing with it from time to time.
First off, I can put everybody's mind at ease and say that the produced code, both markup and css are wonderfully clean. Honestly, in this aspect, Macaw stands way above anything else I've seen. Give it a go and I promise you'll be amazed by the quality of the code!
Unfortunately, I have not been able to efficiently use the tool in a real life project. There are simply still to many quirks and issues with the software for me to be able to efficiently integrate Macaw into my workflow. Simple things, like creating a menu bar, take up too long. I think every professional coder out there with several years of experience under their belt will find that handwriting the code is still faster at this point.
That said, I do really feel the talented guys at Macaw are on to something and if they stick with it, keep making the software better and fix up the existing issues, Macaw will definitely turn into a very useful tool, for both developers and designers alike!
Give it a try!