The iPhone 5 is pretty good, and for most folks, its Wi-Fi is actually very good. It was in our tests. But a lot of people have been reporting in that the Wi-Fi is actually awful, and can't hold a signal at all. This should fix that right up for you.
First off, this isn't the same issue as Wi-Fi data bleeding onto your carrier plan (though the two could certainly be related). This new complaint has been observed by people holding an iPhone 5 in one hand, and an iPhone 4S in another, and the 4S getting full Wi-Fi bars while the iPhone 5 struggles to keep even just one for a few seconds.