The Core

I am after a good fully featured theme that is easy to modify and alter to different designs.

I have been using Headway for a couple of sites which is a great idea, but goes wrong often and has frustrating support (I can't afford to have sites down for a week and then be met with sarcasm).  It might be great one day, but is a work in progress at the moment.

I am toying with the idea of investing in a custom solution, but would be very interested to hear what other WP users are using, modifying or building on?

I use Thesis on a lot of my sites. Pretty easy to work with.

I have also started using Genesis from StudioPress. Lots of child themes and it is pretty easy to mess with the CSS files and get some good customization.

There is also this thread
http://th3core.com/talk/web-development/recommended-themes-and-plugins-for-wordpress/

Thanks for the replies, I'll get looking at those themes. 

@Rumbas - were any of the sites we exchanged links on built using your own?  Maybe you could PM me and let me know?

I was using thesis, but I really like the Genesis themes now.

Those are really nice out of the box. Find something similar to what you want and you just do some tweaking to get it perfect.

Are these paid themes really that much better than what's available out there for free?

I'm wondering the same thing.

The Studiopress themes are nice - still use a blog format as a basis for basic new sites from when they were still "Revolution" themes.

The big frustration for me is that they don't have an inbuilt tab content feature like lots of others: latest,  popular, tags, etc. Seems a normal feature on other themes, not least from Solostream and Woos.

I've spent half the day wandering through ThemeForest.net and am pleased with the killer designs there (sort by "Sales" for best results).  Pick a design and then outsource the customization and you are ready to rock at a great price.