I've had iPhones (3G, 3Gs and now the 4) since the 3G iPhone was launched and I've never yet had a failed or dropped call on O2.
A lot of the dropped calls hype was also down to the mobile networks as well (masts over capacity and network congestion etc). UK mobile networks do tend to work quite hard at keeping up with network demand unlike the US ones (where the biggest dropped call hype was actually coming from).
Yes, I've probably been lucky because of the good quality 2G O2 network in my area. I live in a semi-rural area so demand on the O2 network is likely not as high as in a major city, but on the other hand, there are fewer masts here too.
Dont believe all the hype until you try it for yourself. You have a 14 day return policy with O2 so if the worse comes to the worse and you are not happy, you have a 14 day cooling off period to return your iPhone.