As one of the original purchasers of iThemes' themes almost 19 months ago (and continue to purchase), I can honestly say I've never regretted my purchases. One thing to remember, many people seem to compare them to WooThemes or StudioPress or Press75, etc... But one of the things iThemes does well is create quality business oriented themes that push Wordpress to more of a CMS visual feel.

iThemes doesn't seem to push out magazine themes, video sharing themes, abstract designer blog themes like some of the other mentioned sites. For developers who are cranking out sites for small businesses who just need a simple web presence that is easy to update, there's no way they would choose the Abstract theme or the Urban Elements theme for example. Small businesses are looking for clean, professional looking themes and iThemes seems to fill a niche that caters to small business sites. Plus, iThemes is one of the few (if not only) developers that integrate video tutorials of using the theme and using Wordpress. Many clients love that feature if they are new to wordpress.

Just wanted to add my comments.