I updated my Xcode from 6.2 to 6.3 and now instruments is not working anymore, it is requesting a template and it doesn't matter which one is chosen, it is not able to open it.
It is happening with iOS 8.2 on an iPhone 6+, and with the simulator (ios 8.2 and 8.3).
UPDATE I also updated my phone to iOS 8.3 and tried with Xcode 6.4Beta and I still have the same problem.
Try to run this from a terminal:
sudo chmod a+rwxt /Library/Caches/com.apple.dt.instruments
to fixup permissions on that directory.
Source : https://forums.developer.apple.com/thread/6441
I actually got the same problem with Xcode 7 and solved it by deleting instrument's default base temp directory, and I guess it was in this folder (/Library/Caches/com.apple.dt.instruments)