Yes. I know it for a fact, because I'm using that service now, albeit branded as Clear. I use probably about 40 gigs of bandwidth a month, and have never had any problems.
Again, I'm using that service now, and it's quite close. Comcast cable is still faster, especially for small downloads, but for my individual use the 4g connection is definitely fast enough. Maybe it would be different if I played MMOs, but for downloads and streaming video it's plenty fast.
That's what I'm most curious about. I'll make sure I buy their replacement-policy warranty. $5 a month is worth it, if I'm saving $40 a month on everything else.
Here's my thinking on the topic:
The iPhone 4 will undoubtedly have a smoother and better-designed OS than the Evo's Android. The iPhone hardware will also be snazzier. The iPhone definitely has better apps too. However, the Evo will be much less expensive per month, if its 4g works as well as my 4g USB modem. The 4g would also make an iPad work like a dream.
So it's a tradeoff: Do I want a better phone experience, or to save money and have a great iPad experience. I suspect that if I get an iPad, I'd use it instead of my phone for most things. I'd miss a few iPhone apps I currently use on my iPad touch, but I could use them and more on the iPad, and do them better because of 4g speeds.