Purchase periods last six months and it vests immediately. I sell it the same day it purchases for a guaranteed 15% return (the purchase price discount in the plan).

The increase is counted as ordinary income. If I wanted to be aggressive and go for the long term capital gain tax rate I could, but to me the risk is not worth the ~7% reduction in tax rate on the gain.

My only regret is the low annual cap, which I think is federal law. I would put 100% of my salary into it if I could.