The three main problems are:
AppStore downloads take a long time
AirPlay mirroring jerks
iTunes Match: downloads via the mobile data network can not be deactivated
Downloads from the App Store can be a lot of time at point "Wait ..."
The first problem, which could well do with Apple's servers together, refers to the AppStore: Before starting the download, can be a lot of time at point iOS "Wait ...". Apple itself has not taken any reference to this problem, however, is in Apple's discussion forum already strong (click).
AirPlay mirroring runs choppy on AppleTV with activated Bluetooth
When Bluetooth is activated on the jerky AirPlay IOS devices 4S iPhone, iPad, and iPad 2 3, the image a bit choppy. Apple has acknowledged this mistake, however, already provides the support section of the website in-house support (click).
iOS 5.1 problems and troubleshoots
Mobile downloads from iTunes match can not be deactivated
A vexing problem for non-flat rate Internet users: Even if iTunes downloads over the match network is turned off in the settings enabled, this feature by itself again. This is probably due to the introduction of the possibility that the iPhone 3G can disable 4S iOS 5.1. However, this is a problem that only occurs sporadically, there is a thread at Apple support it. If you suffer from this problem, unfortunately, every time you have to disable iTunes match as soon as you leave your wireless area.
Otherwise iOS 1.5 Despite these problems a recommended update: Not at all, but some iPhone users 4S is the battery life is better and the new features are not bad either. So if you jailbreak on his / untethered jailbreak can do without, which is also under iOS 1.5 in good hands.