I fly economy and BD has been great. I can't think of a better program to have earned miles with. I doubt I'd be gold if it was M&M...and I doubt I'd even try. At least on BD I am sufficiently encouraged to try to earn Gold even if it involves flying a little bit more than I otherwise would have.
There is the trouble of avoiding some of the deep discount fares inorder to earn miles, but they haven't always been that common on my flights.