TL;DR - It was fine, though I know my limits. Test in safe place if you want to try. Biggest issues are condensation making the mask slip down and finding a way to cleanly pack it away.
Although I'm lucky to have a rail trail in my city, I have to ride beside vehicles for most of the trip to get there. This includes getting over some fairly substantial hills which obviously makes me breath quite heavily. Usually I would run out of breath on the second hill and have to take a break, but recently I've also been having coughing fits. This might be psychosomatic as I've learned about how bad exhaust fumes are for your health (thanks NJB). Regardless, I started wearing a cheap respirator from the Orange Box store.
Certainly it makes it more difficult to breath, but for me it's a net gain. No more coughing fits, I can make it over that second hill and the dummy in the massive cope-wagon doesn't bother me as much.
The only trouble I have is packing it away once I get to the trail and the moisture from my breath builds up in the mask so it starts slipping down my face and blocks my nose. I think I'll have to get a more expensive mask to solve that issue.
As a disclaimer, I've been in the cadets and have years of martial arts training, so I know my limits. I suspect some people that try to wear a respirator while cycling might pass out, which is not a good thing to do when the people beside you are putting their convenience over your health and safety.
Stay safe (and healthy) out there.
Is it aero?
Yes, it covers my big nose.