I've been using it for a while, but am not fully satisfied with it. First of all the 'changed' interface is not supported in Firefox, and surprisingly, I get tonnes of errors when I use it on IE7 Beta 2. It only seems to work OK on IE6. Yeah, the toned down interface is much better than the original hotmail, but there's still a lota work left undone by Microsoft.
Yes, you can change back to the old interface if you're not satisfied with the new one.
peace...
aditya