About three weeks ago after getting my Nexus 4 activated on T-mobile, I tried using that sim in my iPhone 4S and only got 3G a few minutes before it changed back to edge.
Today I decided to try again and this time, my 4S has stayed on 3G for the past twelve hours and my speeds range anywhere from 1.5Mbps to 12Mbps. I live in a dead area for most providers. there is about a mile range where data just stops depending on where you are in that one mile area. There is a radio station tower a few miles west if here and I think it is so string that it wipes out cell phone reception. I kind of confirmed that more or less by using an XM radio that had a built in FM transmitter. Certain areas near that tower would wipe out the signal and this happened nowhere else in the city.
Strangely enough, after putting up with 0.00 - 50Kbps on AT&T for all these years that I had an iPhone, I realized that it is the iPhones configuration that was never allowing me to get better 3G speeds. I put the iPhone AT&T sim into my Nexus 4 and configured the APN to one that I found on a website and surprisingly would get on average 1.5-3Mbps on average on up to just under 10Mbps.
T-Mobile is the only network that works perfectly in my house to where I average 17-21Mbps on my Nexus 4. Wish I could get those speeds on the iPhone using that same sim.