I just took the big plunge yesterday (Wednesday) and upgraded my "production" 2.6.3 to 2.7.1+. I had been experimenting with, and customizing 2.7 (More theme) for about two months. On Monday, I had prepared my new "moodle" with a copy of my config.php file, Collapsed Topics, and Checklist. All ready to go. I figured that my current students would not be online Wednesday morning at 7AM, and sure enough, they were not. The last log in was about 6 hours earlier.
I had already practiced upgrading from 2.6.3 to 2.7 on my experimental moodle, everything went fine.
So at 7:00AM, a bit sweating, I decided to do the upgrade. Because 2.7 does more with modifying the database (I think), the upgrade took a little longer, but by 7:05AM I was done! I proceeded to make my necessary modifications to the new moodle, including setting the More theme as computer, tablet, and phone, copying my CSS code into this new install, and adding a few icons to ATTO. By 7:10AM, everything was fine.
When I checked the version of my newly installed moodle, it was something like 2.7, not the expected 2.7.1+. Oops, made a mistake with versions! Quickly, I redid this entire install with the latest 2.7.1+, still no users logged in, and was up to date. About 7:25AM. Well, did a lot in 25 minutes, but this is an advantage of being prepared, and experimenting.
I emailed my students (about 40 this summer) letting them know about the new (yet similar) look, and asked them to let me know if they encounter any problems. So far (it is Thursday PM) only one student mentioned that she has lost the "hover" color associated with course activities. Last night, I experimented with the CSS code (interesting that this feature is not in More of Clean,) made an entry, fixed the problem, emailed the student, and before the evening was over she had thanked me via email.
As I am now using 2.7 in production, I am finding that it is really great.