Region: United States
Phone: Lumia 635
OS: Windows Phone 8.1 Update 2
Carrier: AT&T
This weekend, I unlocked my phone, de-branded it, and updated it first to Denim and then Update 2, aka GDR2.
I am SO glad I did it!
EDIT: I recommend it to anyone comfortable flashing the ROM and doing registry edits. I can't believe how much faster the phone responds to everything. It's noticeably faster (insert "seems faster" reference here) for loading apps, restarting, resuming, everything I've tried so far. Coming back up from a soft reset is way faster than before. This update needs to be available to everyone, in my opinion. It appears to me that my battery life has also improved by a very noticeable margin. Happy days.