I know thesis is one of the major themes available for download. I am curious though...I have a couple of free themes that I regularly use that I really enjoy. Are there any reasons why I should try thesis?
It's a personal choice. You should choose a theme that best displays how you want to display your content to your readers or gives you the style, options and professionalism that you want. I don't like free themes, but that doesn't mean that you shouldn't use them if you have one that you like and are happy with it. If you like what Thesis has to offer, then you should try it. But don't get it just because other people say that you should or because you think it going to give you some kind of SEO leg up, or is better for adsense. Those are all arbitrary reasons that are promotion and sales tactics. I have never used Thesis. It's just a theme like any other as far as I'm concerned. But then again, I don't need a theme with everything all spelled out. The admin options are nice to have, but it all comes down to it's presentation on the front end for me. So far I haven't seen where it is worth the price over any other theme. To me, it's nice, but it's not special. Some people swear by it like it's the second coming of Wordpress. I have never given any theme that much credit because it's all still going to come down to you as a blogger and marketer no matter if you use Thesis or blog on a paper bag.
To me, because it is seo optimised, including XHTML, more strict = more accessable to bot and visitors, if its validated.