In December last year I tried Omega Theme for the first time for one of our in house site. I wanted to have a responsive design that will adjust itself as per the screen size. Initially I was not very much sure how it will be done but within few days I was able to setup a prototype. The result was great but there were still some issues with the images and image sliders. The challenge was to auto adjust the image sizes as well.
The Image auto resize problem was also solved with the help of some modules, though still in dev phase but worked anyways.
This year I worked on Responsive Design a lot and now pretty much confident I can certainly offer this to my clients. So I am going to add "Responsive Designs" in my offerings and guess what? I already have one client who is interested in it and I am working on it now.
So far I only used Omega theme but I will also give other themes a try too.
I am also going to write a detailed blog on how to setup a Responsive Design Theme on a Drupal site. Though if anyone is interested in it can go to http://drupal.org/project/omega and check the documentation. Lot of interesting things are mentioned there.