As many iPad and iPhone users have discovered, there are a number of WiFi issues with IOS 6. Sadly, even more are being reported while the ones discovered early on have not yet been fixed. Frustrated users should be aware that due to these issues their carrier bills my go up significantly and sessions at public WiFi Hotspots may be less than satisfying.
 

One of the issues that will be costly is the data plans that IOS 6 users may be on with their carriers. Many times when running applications that are heavy on data usage (video, gaming, etc), people's phone will switch to WiFi instead of taxing the cellular network. Turns out that when people think they are on WiFi, they are using up the data plan and finding rather large bills as a result. See more here.
 

Connection to WiFi networks also seems to be one of the main issues. Dropping the signal, needing to reconnect. In some cases, users were unable to connect to wireless networks that they had used faithfully before upgrading to the new operating system.

Slow speeds are also being reported over WiFi, as noted in this article from CNET.  As also mentioned in the CNET article, going on the online forums will be helpful for tips and suggestions on how to alleviate the IOS 6 WiFi Woes.