Thesis has been heavily marketed by some of the Problogger / Copyblogger team for some time. I actually have a copy of it and it isn't a bad theme has some nice hooks and can be customised reasonably well.
But it has had a huge machine behind it and there is a strong affiliate program behind it as well (none of which are bad things mind) But every Tom Dick and Kieran seems to have it installed .
At the end of the day it is JAT (Just Another Theme) and when it comes to making the most from any theme you need to build up your knowledge of WP and php and know what to look for.
There are tons of good free themes that can be nicely modified even from the Wordpress.org theme bank and I would think about what you want to do with your WP site before spending your money.