I test drove one today. When I let off the gas, the thing seemed to slow down significantly, but I wasn't paying attention to that much.
I bought the thing. Maybe I'll go nip out and take for a little spin and check the engine braking.
Update: Just went for a spin. Unfortunately not on a steep hill or on gravel, except my driveway.
When I let off the gas at 40mph in high gear, the nose drops and it starts slowing down much faster than I would expect a freewheeling vehicle to do.
When I let off the gas at lower speeds, it seems to coast more freely than it did at the higher speeds.
When I put it in low gear in my driveway, got a little momentum and let off the gas, it felt like it was going to coast as well as it did in high gear at that same rough speed, but I ran out of driveway before I could really tell.
I expected it to nose dive in low gear when I let off the gas but it really didn't.
The granny gear is really very low, but without engine braking, downhills might be interesting.
So it seems like there is no engine braking, but it also seems like it doesn't need it at higher speeds.
Unfortunately the bottom line is that I can't tell you how well it will handle crawling down a steep slope.