As expected, given the number of users that have tried and downloaded the iPhone OS4 or iOS 4, there were always bound to be problems and already dozens of users have complained about their upgrade with some very worrying trends starting to appear.
The most active problems encountered by the users, with the most popular threads here, here and here, point to a very sluggish upgrade process sometimes taking more than an hour to complete, if at all or the fact that backup was not working.
Some users report that their progress bar won't move in iTunes even after they tried to restart their computers with many saying that the upgrade was stuck. A particular user left his iPhone connected for more than 10 hours and his computer, he said, was permanently maxed out.
Then there's the perennial issue of battery life. Because the new iOS 4 now supports multi-tasking, battery life is likely to take a hit, especially on the older iPhone 3GS which doesn't have a bigger battery compared to the iPhone 4.
Likewise the increasing number of multi-tasking friendly applications means that complaints about battery life (here) will become more common.
A minor issue we've come across when moving to the iOS 4 was the appearance of the "other file" category, one which currently occupies 300MB of our iPhone capacity but which fluctuates when importing content. Other users have said that this category has surged to around 4GB on average with one user registering nearly 11GB. More here.
Finally, there's the issue of Visual Voicemail, which some iPhone users reported as being broken. The phone app apparently calls the user's voicemail rather than the Visual Voicemail interface as it is usually the case. In addition, saved voicemails are erased as it is the case for all OS updates.