I had one on pre order with Vodafone but sent it back and cancelled the contract in the first week. The iphone (regardless of network) is a less capable phone that the Nokia I was using before so I got zero signal at home while the Nokia would still make calls.
Vodafone don't support visual voicemail (which allows you to see who's left a message before dialling in to hear the message).
If you want to set up your email using their outgoing survey, be aware it's very overloaded and poorly and they have no plans to up grade it. If you want to use an email address that doesn't have an outgoing surver which will work with mobile devices (like NTL for example) you'll need to route through a googlemail account or such.
There's also no facility (don't know if they is all iphone or just vodafone) to monitor how many texts a single message takes.
I still like the iphone, I'm tempted to have another look when the 4g comes out but I'll look at all networks then unlike this time when I just took Vodafone as they were "my" network. They've left me feeling rather let down and messed around very the whole iphone thing. They really didn't have the customer service systems in place to deal with it.
On the subject of tethering, there's an extra charge on Vodafone if you try to tether your iphone! Don't know if other networks are the same though.