I installed Windows Phone 7.8 over the weekend. It has not gone smoothly. First Zune refused to recognise the phone - it did eventually - and this morning my Nokia 800 locked for the first time ever and I had to google a soft reset which worked thankfully, from here: "Press and hold the Volume down and Power buttons until it vibrates, after that you have to release all the keys then phone will vibrate 3 times." This happened while playing a podcast. I have had issues with the media player in the past, chiefly it not maintaining the state of where I was in the podcast, but nothing this major.
Here's hoping for few further issues with 7.8 which, by the way, doesn't seem to add a great deal to 7.5. So I might be googling how to rollback to 7.5 in the not too distant future if the problems persist. Not to downplay the complexity of any development when Microsoft is effectively supporting 2 platforms but 7.8 has been so long in developing one would have though they could have got it right. Too many issues and my loyalty to Windows Phone will be severely tested.