For my part, it takes me roughly two hours to:
* Copy all the static assets to their Rails-appropriate locations
* Take the "demo" pages and get them to display reasonably as Erb templates of a Test controller
* Convert the HTML to Haml
* Move content to layouts/application
* Break each feature in the demo out to a partial
There is no other two hours I can spend that gets me as much UI functionality, so I'm a fan.
I have done this three times, and each time found the work of integrating a Themeforest template to be easier than integrating work from graphic designers.
I am careful to work with templates that are HTML5, CSS3, jQuery, and that use a grid framework. I can imagine there are many hundreds of themes there that fail one or more of these checkboxes, and that might thus be a nightmare to work with.