If you’ve upgraded to 2.7, did it go smoothly? Any gotchas that we should know about?
I have done it a few times now for some of the instances I manage over here and the only thing that was a “gotcha” for me was remembering to clear my browser cache as there are a bunch of new UI elements that don’t load if you forget.
I second Ty’s comment. I have been a part of many upgrades so far and the biggest gotcha is the browser cache. The easiest way to identify this is the logo. In case it hasn’t been noticed yet, our logo has been refreshed 
If you upgrade to 2.7 you might see the old logo still in the upper left. In most browsers, you can hit Ctrl+F5 to do a Cache Free Refresh, which tells the browser to load all pages and resources fresh and clean from the server. The interface will update at that point.
Also, if you aren’t sure if you are on 2.7 yet, click the ? mark in the upper right corner and hit the About screen. It shows your version and build.
2.7 was my 3rd upgrade on CM1 and all have gone smoothly. One thing I have always found though (and I’m not sure if this is a bug on our system): Their upgrade directions tell you to stop the CM1 server before starting the install. However on our platform if you do this, you can’t get past the login screen for the installer. I’ve found that you have to leave the services running, and start the installer. As soon as you enter the credentials and press next, you can stop the CM1 services. Other than that just remember to clear out your cache after every upgrade (and, more importantly, remember to tell your users the same).
As the others have said, the biggest confusion came when I logged in and didn’t see any of the new features. This was solved by clearing the browser cache.
The only other thing that slowed me down was that – since we use the Active Directory integration for our authentication – I needed to make sure I had a local-to-CM1 admin active to do the upgrade. The installer asks you for that username/password.