We've also been using the iPad mini 2 iOS 8.4 update for about a week now. Like the iPad Air, we had been getting outstanding performance out of the second-gen iPad mini and were hopeful that iOS 8.4 would provide the same.
So far, so good. We've been using the tablet extensively over the past week or so and we haven't run into any problems with its battery life, with its various connections, with its user interface, or with our core group of applications. It's the same as iOS 8.3 and that's a very good thing.
And like the iPad Air iOS 8.4 update, we don't anticipate any changes with the update's performance. The iPad mini second-generation and the iOS 8 update have been agreeable and we now have our sights on Apple's upcoming iOS 9 update.