I don't have one but I've used it at the Microsoft store. I opted for the Surface Pro because with the Ativ, you'll usually need to bring it around with the keyboard dock, which adds a lot of weight to the device. Further, it doesn't have a built in kick stand, so you can't prop it up whenever you like unless you put it in the dock. Another problem is the dock doesn't have secure connection and often disconnects with the slightest movement, which would be really annoying. The ativ is also 11.6 inches which isn't the most pleasing size for a tablet, where as I find the 10.6 inch surface is a lot more pleasing to use especially as a tablet. The build and quality of the ativ I also find quite cheap and the usb ports and such have plastic covers which I find annoying.
With the surface pro, I don't need to buy any folio case or anything since it has a kick stand and a cover for the screen, while it serves as a keyboard at the same time, making it considerably lighter than the ativ when docked.
IF you're looking for something like that, I think the best viable options are the Surface Pro or the Acer W700. If I wanted something like the Ativ, I'd probably just opt for an ultra book
Yeah, as much as I wanted a Surface Pro it was the storage that ultimately changed my mind. I ended up buying a Lenovo Thinkpad Twist ultrabook that has a hybrid storage system with a 500gig hd. Plus it was considerably less expensive , $670 on sale, and I felt the Pro might make my RT somewhat of a paperweight (although probably not considering the battery life of these Core I5 devices).