Interesting thread here. I set my phone to Sprint-only, no roaming and I haven't hit any areas yet in NJ, Philly-metro, NY, CT, MD, VA, NC, FL, and MA that I didn't get Sprint service. I did not get much Sprint service in northern ME, but I checked for roaming signal and didn't get any VZW there either. I think some smaller, regional provider showed up but I never made a call. Data there wasn't an option.
My lesson on roaming - I can generally stay with Sprint only. If I can't get a call through, it's easy enough to toggle roaming on and search for another signal. If I'm that far from Sprint service, I don't expect much, if any, access to data.
As to the post of why Wimax matters? Well, there isn't a day when I'm not using my phone online to respond to biz or personal email, check biz accounts, or other items I need to do. If a call comes in, I don't have to stop what I'm doing. If I'm asked to look something up, I do it right then. That matters because people really get jazzed when you tell them they don't have to wait while you retrieve the information. It also matters in major metros where Sprint has strong 4G signal and coverage.
The way I get in and around NYC, Philly, and Boston is much better now with 4G. I can talk to people as I'm getting to them.
Of course, it's tough on battery (as all multi-tasking is). That's what extended batteries and portable rechargers are for.
I've had VZW friends brag about speed. Yes, they were faster. But it wasn't practically that much better. Just because you can DL/stream faster doesn't mean you can watch the content any faster. My speedy VZW friends are always conscious of data too. I never win the speed test, but I always win when it comes to continuous use. I never see VZW friends walking around using Google Maps/Layars/etc while talking to others while driving/walking to our meeting point. I do it all the time thanks to Wimax.
As for TMo - they're very good in their strong coverage areas. If I lived in downtown Philly, I'd probably use them. TMo doesn't hold up well for traveling though. And those speeds they show don't hold up across their entire coverage footprint. They're good, just not great. If they were, people would be signing up for them in droves and they wouldn't be an AT&T acquisition. And IF TMo gets swallowed up by AT&T, expect the speeds to drop significantly. Those iPhoners would love to be able to have another pool of towers to choke.