At the risk of sounding linke one of those tedious "it worked fine for me" types:
It worked fine for me.
Same phone, but all updates were done from Win 7. One update to prepare for Mango, accompanied by a little excitement about getting Mango, then a relisation that it was to prepare for Mango, when the next update came down straight afterwards. I can't really tell you how long Mango took, I went to the pub and when I was back the phone was working fine on Mango.
I quite like it, most of the differences are not initially obvious, but they are there and it does make for a nice OS to use.