I haven’t, but millions of people do. My parents do, for example. They get their cell service through a MVNO called Consumers Cellular, but that is because Consumers Cellular is buying time from AT&T. So the actual conversation is on AT&T despite the name on the bill.
This is really no big deal – Sprint resells theri service through a bunch of other providers (Virgin Mobile being one).
I’ve never understood the reasoning behind doing this – why put another level between you and your carrier?