It's a high risk, high reward travel in a zombie apocalypse.

You're body is exposed to grabby biteys, it's quite easy to take a tumble (roads won't be well maintained, zombies won't move out of the way), you are limited by your health (injury and exhaustion), and your carry capacity/content is limited (can't carry a long or heavy thing easily)

However, as you said, quieter than most other modes of transport, much faster and more energy efficient than foot, don't have to worry about fuel (other than your bodies).

I think I'd use them, but I've assed off enough times downhill mountain biking and a few times in town (bus cutting blind corner, people being careless getting out of cars) that I'd be very wary