I carry both an iPhone 3gs and a Blackberry around most of the time (I'm currently justin.tv's iPhone developer). I find the iPhone pretty much useless around San Francisco because of AT&T's abysmal network coverage - I tend to just not bother and use the Blackberry instead, even though it has a worse web browsing experience (at least it always works! I haven't found a place yet that doesn't have good Sprint coverage).