What I've noticed so far:
** Battery life is tricky: sometimes I woke up and the iPhone (5s) is at 100%, and some other days it consumes 20% overnight with no mails, calls or any activity. Set it up as a new phone.
** Weather app: seems off-center. Letters on the left are cut in half.
** Most annoying = random loss of keyboard sound clicks and app notifications (especially WhatsApp). If I receive a message, mail, whatsApp or whatever while the phone is charging, I don't get sound notifications at all.
** Animations seem a bit slower., but I don't find this as a big deal for the moment, considering it's a beta.
** Interestingly, I've had no resprings or random reboots (happened a couple of times while on iOS 8.4).
I hope performance and stability improve with further betas.
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Mark