Anecdote!
Anecdote alert:
Just switched from a BlackBerry Bold on AT&T to a Palm Pre Plus on Verizon. I live in Northern Virginia (Washington, DC metro area).
Over the last 6mo-year, I noticed AT&T's service quality degrading very quickly in my area. More dropped calls, more trouble getting data to work, etc. My wife and at least a couple of my friends had similar/identical complaints on a variety of AT&T phones, including the iPhone.
When data worked right, it was speedy...generally...though it tended to have a lag. I'd click to load something, it would pause for a few seconds, then load it pretty quick.
Now, on Verizon, virtually no dropped calls and much more reliable data in more places. Data downloads are noticeably slower than they were on AT&T, but there is no starting lag (click to load something, and boom it starts loading)...so in real world usage (surfing the web) it seems faster, even though the actual data throughput is less.
As always, your mileage may vary...but in my area, I certainly didn't see AT&T service improving over the last year... quite the opposite. I had been a happy AT&T customer and defended AT&T when people criticized them...but they got so bad so fast that I had to eat the early termination fee and jump ship.