DM here. Was a PM for many years. So far, I have 28 DM flights (3/1/10 on). My story is as follows:

Positive:
Twice I was upgraded when I would not have been as a PM.
Once I was met at the plane and walked through customs at JFK because I was DM.
Feeling a little special.

Neutral:
Once I was not upgraded because another DM bought his ticket before I did. (I consider this neutral, but significant, as it shows being a DM is not that exclusive and does not put you first in line - just ahead of non-DM.)
As a PM, I would have been upgraded anyway the rest of the time.

Conclusion:
Other than the "feeling" of being special, the real benefits over PM are just not there. Real benefits would be:

VIP system wide upgrades - not just domestic or limited int'l
Guaranteed coach seats on fully booked planes
Free drinks and snacks in coach.

I might add, at AA they call those real benefits being Executive Platinum.