I wanted to design a wordpress website 'from scratch'. Thematic provided me with the bare bones skeleton to do absolutely anything I wanted with my custom theme. I spent three months refining the design until it felt perfect (have a look at oldmankit.com), and at every step of the way found thematic did what I needed it to do.
What made me write 'blown away' above though wasn't the power of the theme, but the humanness and helpfulness of the support. The people behind Thematic seem to really love what they do, and are just so very very helpful.