the iphone used to be pretty bad with reception. Mine is the 3g and it was a shocker on o2 for 3g (didnt exist) signal strength.
Vodafone has good comprehensive coverage although i have never used it on my iphone.
Ive been using mine on three, i havent had a problem with the 3g signal at all, i travel a good 18 miles in the car to work and i can be on the phone for the whole time and not drop a call (blue and me integrated on my car before people start having a hissy fit about driving and talking on a mobile) and thats from driving in the middle of nowhere to driving through towns and villages so i wouldnt say the iphone is rubbish with regards its signal, its more down to the network.
I have been with all networks in the past and i would have to say that vodafone is better with coverage in terms of that its solid indoors and out, then i have to admit that three even though they where absolute pants a few years back have really pulled there socks ups and got things sorted. Plus they are bloody cheap on pay as you go, great little internet bundle for surfing on the go thrown in every time you top up.
So this is my little list regarding networks and 3g coverage.
1st place Vodafone
2nd place suprisingly three
3rd place orange
4th t-mobile
5th o2 they have a very strong 2g network but totally non existent 3g coverage where i live/work/play etc.
it will be different for everybody, i wouldnt worry about the iphone regards signal its no worse than any other phone, your best off checking coverage of each network and working out what is best for you. For all i know you could be living in a excellent 3g area for o2 but a non existent one vodafone.