We migrated all the exchange users from 2010 to 2013 last night. All phones work fine apart from iphones.
They take roughly 5 minutes to load the OWA web page. Phones that managed to keep the activesync connection still sync but slowly and new ones fail to verify.
All IOS devices are up to date.
Running exchange 2013 SP1, we have roughly 80 ipads which have no problem.