I had an issue when I first activated the phone, I could only open one web site and then I would have to reset wifi if I wanted to connect somewhere else but after rebooting the iPhone problem disappeared and never happened again so far.
I'm having problems where my phone seems to use LTE even when I'm supposedly on wifi.
2.4GB in one week when I have never even reached 2 in one month is more than the difference in speed. I've not used my phone much differently than I ever have- I'd expect a slight bump this month but there's no way I've used 2.4GB when I'm almost always on wifi. (I'm on Verizon by the way)
IPhone 5 has issues connecting to Specific WiFi Settings, e.g., 5 GHz. This problem manifests in unreliable connections, slow connections, Network Timeouts and the user unknowinlingy using their cell data. Apple stores have set aside iPhone 5 specifically for exchanges. However, Apple has not acknowledged the issue.