Even two iPhone users can have completely different experiences in the same vicinity. It's hard to gauge unless we know what you are working with. That said, my friends w/ T-Mo have pretty good experiences in downtown LA. In other parts of Cali (mostly bay area), my friends with iPhones/BBs split between ATT & Verizon have different types of complaints.
Personally, I'd go over to Howardforums.com and get a sense of what others are experiencing where you use your phone. I am guessing you are an ATT customer ;). Some folks even discuss time of day/block to block/turn by turn while driving/ use of certain handsets/providers. That sort of micro-data may help you decide if its truly the phone or perhaps the network.
I currently use AT&T near Downtown, Los Angeles area. Most of my grievances are due to my phone (which I am certain about). My contract is up and I was wondering if I should switch. So I wanted a general consensus about which is the best network provider in Los Angeles in terms of 3G connectivity and call quality, i.e. Data and Voice network.
Any inputs would be most appreciated. Thanks for your time!