I just pulled mine out to check... been using the dt366's almost exclusively. Really only use the 810 on the porch.
I haven't had a MD with UI1 where I used the interface on the MD itself- I've defaulted to an orbiter, so I don't know how responsive a MD is to inputs, but I have been exceptionally impressed with the responsiveness of the win orbiter. It seems that as soon as I release the mouse click the next page pops up and is populated right away (802.11g network).
The 810 is close to this- I would estimate 1/4 to 1/2 a second after touching the screen for the next page to display. Selecting an artist adds another 1/4 second to the time (maybe 3/4 a second to display an artist's list of albums). From the 810's home screen to a ready-to-use LinuxMCE UI1 screen takes about 12 seconds (using the second hand on my watch). Some of this delay could be from the network- my nas is on my external network (ducks head waiting for "why do people keep doing this")
There is a button that I assume is supposed to have the same effect as alt+tab if you press it once, or the "show desktop" button if you hold it down. This is a quick and easy way to get back to the OS's home screen. Unfortunately, holding the button down or even just pressing it always results in the error "LinuxMCE Orbiter is not responding. Close application?"
When starting the orbiter program there is a setup screen that you have to click through. It is not intrusive, and is the same screen that you use to tell the 810 where your core is located (by IP) and what your orbiter id is. I can't remember off hand, but I think you have to configure the orbiter in LinuxMCE ahead of time, the setup is not like that of a win orbiter or a 366. It's not hard and only required the first time you connect the 810 to your setup.
I do not find the buttons too small to use with my pointer finger.