I've been using mine since launch day and the only issue I've had was that our IT folks had used a self-signed cert for our Exchange server so I had to jump through some hoops to get the cert installed so I could connect. Other than that, it is fast, fluid and responsive as you say. I had already used Windows 8 on a desktop PC with a touch screen (and disliked it very much, and still do on a desktop PC without using tweaks to shove TIFKAM into the background) so I knew the shortcuts and gestures and once I used them on the Surface I had that "A ha!" moment. On a tablet it works. I imagine the UI works on a phone as well, but I haven't had more than 5 minutes with a newer Windows phone so I can't really say.
No audio issues other than the speakers are a bit soft for my taste (you aren't going to be playing music for a room full of people with this thing, for sure), and no issues at all with the touch cover.
Oh wait, I forgot - I do have one other issue with the unit. I am having the problem with the Windows logo on the back wearing off. Meh.
Now, lest those anti-MS folks think I'm a Windows fanboy, there are some design issues with the device that I have encountered (and a couple that make me go "how could they have missed that?).
First, both the front and rear facing cameras are 1MP. Seriously? Why in the world did you even bother with such a wimpy camera?
Second, the touch cover partially obscures the rear facing camera when folded back.
Third, the kickstand has one and only one position so you can't choose a more shallow angle for use, and the steep angle isn't appropriate for all of my normal uses.
Fourth, the SD card doesn't become an integral part of the file system - that is, you can't install apps to it, and you have to jump through hoops to get it to automatically store media and other files onto it to take some of the load off of the built-in storage.
Fifth, although the magnetically attached power connector is neat, it doesn't self-align usually, and a couple of times I've had to fiddle with it for a bit to get it to connect.
Sixth, I find myself inadvertently adjusting the volume when manipulating the device. Either the position of the rocker needs to change or the amount of effort needed to activate the switch needs increased.
Seventh, the kickstand only has a cutout / notch on the left side of the device to make it easier to open, it should really have one on the other side too.
Minor design issues aside (and I expect the next revision will correct most of them), the build quality is very good.