The past few days I've been bringing in my new WIFI-only iPad and trying to use it tethered to my iPhone 5. It's been a frustrating experience so far. I've found that it works OK when the iPad is first tethered to the iPhone, but shortly thereafter the internet connection seems to drop out on the iPad, even though the iPhone is still reporting it to be connected OK.
I finally moved my cellular provider from O2 to Three, because Three has an unlimited data + tethering plan. So far it's been working really well. I brought in my wireless-only Nexus 7 to BNP Paribas' office today, tethered it to my iPhone, and been able to access my email, notes, and other online services quite well. My Nexus 7 is paired with an Apple Wireless Keyboard, and it actually makes a very usable environment.