>> I'm not totally sure I got this completely right, but here's what I get
for S&P500 monthly for the past 25 years, Jan-1985 thru Dec-2010.

....

So you get an idea of why people here aren't terribly excited about margin leverage. You made an extra $2200 and the broker made $13K.
Or you made $8400 and the broker made $7K. You took on additional risk but the broker got the lion's share of the benefit. <<

This also doesn't account for the possibility of losing your shirt with a margin call in any of the three market crashes in that period (1987, 2000-02, 2008-09).

#29