I've been to 2 theaters and have seen 3 movies...
First one was the El Capitan in Hollywood to see Brave, which was the first (I believe) movie to be encoded in Dolby Atmos.
The second theater I went to was the Cinemark at 3800 Dallas Pkwy,
Plano, TX. The first time was for The Hobbit: An Unexpected Journey. The second movie was Pacific Rim.
One thing that I noticed while at the El Capitan is that the theater was playing back at reference volume, which was loud and awesome in general. Things that I noticed was a more enhanced surround experience, and not just from the speakers above the seating area. There are, generally, more discrete surround channels. One thing that I remember when I saw Brave was that at one point there is rain coming down and it is falling on some wood planks that are over top of a cave or something, and it sounded just like the rain was coming from overhead. Outside of that, it was more subtle for the rest of the movie as well as for the other movies, but it did create (like I said) a really enveloping surround field.
Any questions in particular that you were wondering about?
For others wondering, here is a link with a map of theaters with Dolby Atmos sound: Dolby Atmos Theaters
If you see a circle with a number in it, that means that there are that many number of Dolby Atmos theaters in that area.