Been a sprint customer since the late 90s. Started with a Palm Pre, upgraded to an EVO 4G (WiMax) and am now on a Galaxy S3 LTE.
The WiMax coverage for the EVO was primarily in the larger areas as people have said but I knew that before I bought the damned phone; I'd have felt silly buying a "high speed data" phone without researching whether the product was designed to work in my area. ;) WiMax, where it was available, was always very fast and connections were reliable enough that I never noticed anything untoward.
My S3 has much more limited coverage (which I knew) but is even faster in-zone. I primarily use 3G but I'll admit to not being the type that uses my phone as an entertainment console, so the occasional surfing I do to look things up while shopping or browsing weather radar or news while out and about works like a champ so far.
So far, I can't complain about Sprint. I haven't had any trouble with them jacking my rates or failing to grandfather in the All Everything All The Time plan I've had since 2002 or so. Based on my experience, I suspect this is mostly due to being both cognizant of realistic expectations for coverage and vigilant about letting the sales reps not screw up my paperwork.