- find where the cart area is,
- drag item to the cart box (assuming it's highly visible and has a nice chunky landing area),
- release the mouse button.
In such instances I think it's excellent, and if I was a clothes retailer I would be very tempted by these solutions. It would be even better if I could take a head shot of myself to load on top of the clothes dummy to get a better idea of the outfit I was designing - because I hate the thought of ordering clothes online only to try them on after delivery and not like the look.
But I can't help but think it'd be useless for any other type of retailer that's not in the clothing business? I can't imagine buying shrink wrapped box of software from citysoftware would benefit from a drag and drop site. Same for a remote controlled toy on a hobbies site. Good reviews, high quality product presentation, videos and comprehensive specifications get my attention when I'm buying such things.
My prediction - drag and drop carts have got nothing over "social" carts (the prediction - facebook is going to turn into a kind of social cart, I agree with scobleizer on that one) other than a little wow factor, unless it's engaging the customer into an activity such as "trying on clothes".