Biggest problem with cotton wood (around here anyway) is that the bark collects quite a lot of dirt and is really hard on chains. I don't seek it out and cut and burn it only if it is really easy to get.
Cotton wood stinks, is miserable to split, hold too much water when it's green, and doesn't keep very long once it dries. If it is the only thing available, why not?
I had access to a lot of elm when a local cut down two very large trees last spring and to tell the truth, these were only a small step better than cotton wood. The rounds actually kept and burn pretty well, the split pieces are very dry and burn quite quickly. It also leave a lot of ash, and is prone to form a lot of klinkers.
Mark
Mark