when i was a mechanich we serviced some trucks for a local propane deliverer that ran on propane, even after 300,000 miles the exhaust manifolds were clean enough to eat off of. And with the natural gas- MODOT ran some engine oil analysis on their viehicles and the recomendation was to change oil every 25,000 miles because the worse engine in the fleet showed signs of oil degredation at 40,000. It is some clean burning crap.
I still like ethanol though, primarily because its a liquid, and liquids are easier to store and transport than gases.
Yea you can't cold start a 100% ethanol car, but we can find a way around that. Even if it means having a seperate fuel tank for straight gasoline.