If you use wifi with your iPhone, try a quick google search for wifi issues after upgrading to IOS6. I regret upgrading my iPhone and iPad as they now both have difficulty in connecting to any wifi router unless within a few feet.
Some have definitely reported WiFi related issues ranging on different phones, some iPhone 5, some iPhone 4, etc., and even going by different OS versions, some just iOS 6, some iOS 6.0.1, some iOS 6.1. In the end, just like various issues here and there, this can be something that can affects some people with some phones on some iOS versions...but it's far from anything that's really widespread or has anything particular that would indicate the issue would or wouldn't be there. Definitely a fair warning, but also far from anything that's a given really.