Many people are having severe issues. Full signal strength with drop outs on the 4S and 4, making the wifi unusable. Some have the wifi greyed out completely.
On the iPhone 5, some people only have a good connection when in the same room as the wireless router. Turn a corner, and it dies, even though it still has full bars in the connection.
Consistent drop offs with full bars.
Everyone had zero issues with the same devices (the 4S and 4's) on iOS 5. This all started with 6.0. Apple pushed out one wifi fix, and it didn't fix everything.
Not everyone is experiencing this, but there are hundreds of posts on the Apple forums with these issues. It effects iPod touches and iPads as well.
DFU restore doesn't help. Jailbreaking doesn't help. Resetting network settings doesn't help. Changing setting on the router doesn't help. Some are temporary, but the problem always comes back. It's a software bug, that needs fixing for this who have issues.
Just a few...
This one with 156 replys...
https://discussions.apple.com/message/21176656#21176656
iOS 6.1 wifi issues