I bought Thesis some years ago after getting tired of having free WordPress themes break all the time. I must admit I haven't delved too deeply into the customisation of it except for the several screens of design/theme options within the WordPress dashboard itself and a little bit of CSS tweaking. It does have a bunch of custom fields built into the post options for things like SEO optimization.
I know they've just released Thesis 2.1 and it appears to have some interesting new features.
The only thing I would say is that it is important to think about what you want it to use for. When I first bought it there was a "Developer Option" (I paid more to use it on many of my own sites). If you're using it to develop for other people's websites keep in mind that in addition to the purchase of the Theme there are also licensing fees.
Client Site Option $40.00
This option allows you to place Thesis on one  client site.
5-site Client Site Option $180.00
Save time AND money with the new 5-site Client Site Option. Or, if you're really reeling in the clients, save even more cash by purchasing a 10-site Client Site Option.
10-site Client Site Option $320.00
Save $8 per site by purchasing a 10-site Client Site Option.