Lots of chatter on the net about iPhone 3G problems - dodgy apps, apparent firmware problems with responsiveness and GPS, and of course that rather lacklustre battery life. My experience is limited - Apple promises to make good on that, thanks Ved - but not inconsistent with such reports.
The one that's most interesting, because it can be measured, is lack of welly with 3G reception. It's famously difficult to characterise this because of issues like local blackspots, shielding from buildings, and network congestion - but according to this Apple support thread there is the chance of getting something approximating to actual data.
If you enter *3001#12345#*, you'll go into Field Test Mode, where one of the indicators is the signal strength in dBm, or decibel milliwatts. Anything below around -100 dBm is not good -- radio engineers, preferring consistency to clarity, work in negative numbers where 'higher' is in fact lower, so -80 dBm is a better signal strength than -105 dBm. (For reference, -90 dBm is one picowatt, giving you some idea how little power we're dealing with here).
if you're getting -100 dBm in an area where you'd normally expect decent O2 3G coverage, you have interesting information. It's even more interesting if you're on another UK network, but those results may take longer to appear.
Do let me know what you find. It's unlikely that the calibration on the iPhone is good enough for truly consistent results across units, but they should be close enough to get an idea.