I've now had the time to sit back and evaluate my three 3G experiences here in the USA and two in the UK.
In a nutshell, AT&T's HSDPA whoops butt over Verizon, not so much in speed but in network transport, and is better than Sprint for many of the same reasons.
In speed tests I've run, Sprint comes out way ahead and for the casual user may be a better deal than the other two, and while Verizon has the best footprint, for the most part, having a combination of Sprint and AT&T pretty much gives me all I need when I need it.
In the UK I've used both T-Mobile and 3. I tend to like 3's pricing better, five pounds a month vs. a pound a day for T-Mo's web n' walk, but both provide tons of coverage. The 3G, HSDPA coverage around London is fail even, but its the value of the service that makes it a real winner.