I think the inclusion of Data Sense and the Pro Cam update are two big features worth whatever issues the GDR2/Amber update causes. That said, AT&T does not want another .1308 on their hands here. They don't want to push an update out that will create connections issues, battery issues or something worse just to get two new features into people's hands and then have to issue an emergency patch in 2 weeks just to get phones working again. This was the route that T-Mobile tried with the 810 and - although I am very happy with the leaked update - too many people were suffering 3-4 hour battery life due to the new features. Of course, the easy fix was to turn off the new features like Glimpse and Double Tap to wake, but that isn't a solution for those that wanted to use those features. I also had to turn off NFC and Tap to Share to really get great battery life, but that's besides the point.
For those that are saying AT&T is holding the update to get you to upgrade, you couldn't be more wrong. AT&T would rather have you under contract for 23 months, 29 days, 23 hours and 59 minutes before you lock in the next 2 years. Providing updates to your current device will keep you with them longer because you feel that you are being taken care of, not forced to upgrade at full price every year. The Lumia 925 just hit AT&T with GRD2 on board... The 920 will see the update as soon as they can, they really don't want to give you handset that they lose money on instead of a simple software update to the one they already paid the manufacturer to give you.