When I fist walked in on the WordPress scene, I was immediately impressed with Thesis. So much so that I determined to purchase Thesis as I learned and experienced that Thesis was a, maybe the, pioneer with Frameworks and easy DIY WordPress customization. However, having built sites with Thesis, I came across StudioPress right before Brian and Nathan first created Genesis. Having been impressed by its few themes then, when I saw Genesis come out, I had to have it. Not only did I have to get it, I changed some of my sites over from Thesis to Genesis, and I became a Pro Plus Package Member, probably the best Premium Theme package. I haven't been the only ones to notice StudioPress. Others like Chris Brogan and Matt Mullenweg (founder of WordPress) took notice.
So what makes StudioPress so awesome?
Best in Class Premium Framework and Child Themes
StudioPress has an ever growing theme library and supports developers to build their own child themes to be sold separately from StudioPress at places like ThemeGarden. So StudioPress enables other designers and developers and encourages further entrepreneurship instead of bottle-necking any further innovation and creativity that Genesis could possibly spawn. This also speaks to StudioPress's original goal in creating Genesis, which is vastly different from Thesis's DIY business model, as Brian Gardner mentions here. Besides many obvious best-in-class themes, one of the ones that I am most excited about is Prose, which comes the closest to "competing" with Thesis head to head with the added benefits of a framework and a customizable child theme. However, since, as I mentioned, StudioPress is not, has not, set out to directly compete with Thesis (primarily because of its different business model/goal), Prose enables developers and designers who use WordPress and StudioPress's Genesis framework for their clients to provide a more DIY theme, or a theme that can be easily duplicated, or a theme that allows team members with less CSS skills to do more design work than normal. Finally along the lines of theme work, they do not hold back in passing out compliments to other web designers who use Genesis or any other of their child themes as they showcase them on StudioPress.com.
Best in Class SEO and Security
Besides some awesome Theme work and development, StudioPress quickly became SEO conscious ensuring that their theme loads quickly (comparable to the quick Thesis and faster than WordPress's Twenty Ten), is SEO "compliant," and is SEO friendly. Furthermore, as Adam pointed out in his comparison of Thesis and Genesis on security, Genesis "has been the industry leader since its inception." Furthermore, recently StudioPress and Copyblogger merged together. Thus with ScribeSEO and Premise, StudioPress, now StudioPress by Copyblogger Media is poised to set sail to the deep blue ocean.
Best in Class Support Group and Developer Community
Besides supporting their themes and framework in the support forum, which is bar none one of the best around, they support developers with their developers section, where anyone can find community tutorials by community vetted Genesis MVPs. Furthermore, there are priceless developer resources such as (1) introductions to hooks , child themes, (2) reference pages regarding filters, shortcodes, and hooks, and the awesome visual markup guide (because everything needs a visual guide). Personally, I believe the developer's community fills a much needed gap. I see so much more potential for this innovation that has yet to be tapped.
Summary
Finally something needs to be said about the StudioPress staff, mainly Nathan Rice, the lead developer of Genesis (as well as the creator of other prominent themes/frameworks). Besides being a stellar individual, he is an awesome theme/framework WordPress developer. He has also contributed several Genesis/StudioPress specific plugins (as well as WP-Cycle, perfect for theme building!) including Simple Hooks, Simple Edits, Social Profiles Widget, Simple URLs, and Simple Sidebars. Probably one of the coolest was the SEO Data Transporter which can translate the SEO data to and from Genesis as well as to and from Thesis to enable anyone to switch to Genesis as well as switch from Genesis easily (which once you come to Genesis, you won't leave). Another really great thing is that they are not hesitant to mention plugins made by StudioPress developers such as GenesisConnect.