We had a natural gas powered Rinnai unit installed in our house when it was built 12 years ago. We've been very pleased with it, and I would not want to go back to a conventional tank system.
Ours is mounted on the outside wall of the house, and the only time it has frozen (actually just slushed up a little) was when the temps got down in the low single digits. Leaving a faucet running has always been adequate to keep it from happening again.
