This has been the case for a while. Sprint had a reputation as a second rate carrier for a while. They would sell phones to just about anyone when the others required good credit or a large deposit. They didn't advertise their smart phones well until maybe the Pre.
Contrast that with Verizon who has done a great job of cultivating their reputation as the most extensive network. This is not just typical marketing BS because they really did buy up lots of regional carriers and have extended their reach all the way into a lot of rural areas where Sprint, ATT, TMo, etc. are forced to roam. The others seem to concentrate in cities, suburbs, along highways--the places that you will find 90% of people at any given time. Granted this is no help to you if you live in a rural area and have spotty coverage.
Thankfully, Sprint roams for free on Verizon so you may not have blazing fast data but you should always have a signal. The only times I ever really roam are when I go into the DC Metro (Verizon exclusive contract in the tunnels) and when I visit the girlfriend's family in rural PA. At their house I get a measly bar of Sprint so I usually turn on roaming because VZW has better coverage.
Generally, 3G coverage goes Verizon>Sprint>AT&T>T-Mobile. Check the coverage maps because more important than any must-have handset is decent coverage. There is no point in buying a cutting edge smart phone that can never seem to get service so figure out what carriers serve your area, then compare price for similar plans. Lastly, figure out what handset to buy. They all have decent offerings but sometimes you want to wait if you know something big like Evo is on the way in the next couple of months.