I bought new AVRs (Pioneer VSX-53's) and the N66U at the same time. Have had very inconsistent airplay connections and its only successful about 33% of the time. (Although I also have a Yamaha HTR-4065 AVR that Airplay works all the time on). I was going to bring the receivers back this week but I'll see if the new FW makes a difference.
Update: Yeah! After updating to this firmware, the Internet Radio and Airplay seems to be working fine now on my AVR's. Also, my Samsung Galaxy S3 phone is now able to update/load apps from Playstore. This was a problem related to a Sprint Airave that I have connected to the router due to poor cell coverage from Sprint.
Thats great news! This firmware has been just awesome! I'm waiting for merlin's 270 firmware to get out of beta so I can use. I would recommend his firmware because of a few extra features and bug fixes that he implements. To the casual user you might not need the extra features but its nice to have just incase.
Question to RMerlin, what made the 2.4ghz so stable now? is it the new wireless drivers? will they be in your next release?