I actually have! Hartford the state capital and 20 minutes up the road from me has a team in the second tier and one in the fourth tier (note they don't have promotion and relegation, you're just stuck in they leagues forever. Weirdos). 
Hartford City in the fourth tier, a completely amateur side, was a fun wee game played on a college team's pitch, so as you'd imagine it'd be - sitting in bleachers, a couple of hundred folk in attendance (and we still managed to bump into another Scot!). Was, I'd say, probably about the same level as our Lowland league but with the fitness/athleticism turned up a notch.
Hartford Athletic in the second tier are pro and have their own stadium. They would be I'd say a good match for lower Prem/higher Championship sides. But at that level as you'd expect the "match day experience" is more of a factor, there's a live band before the games, a food court (yes, court!) as well as a bunch of stuff for the kids. The main appeal for this kid was that I could buy a few cans and actually stand at a game mind