I wanted to replace my first (rushed) banner, so I just uploaded a 3rd one, and my 1st one became disabled. So that part of the system seems to be working
High rep users seem to get two slots.
Also, I just discovered by accident that if you go here, it shows you the click/impression count:
http://www.gamedev.net/index.php?app=nexus&module=clients§ion=purchases
Apparently I've had 26793 impressions and 115 clicks.
[edit]
Given GDNet+ is $40, that's 0.15c per impression, currently, but will continue to go down as the ads are displayed more and more.
AFAIK average CPI costs around the intarwebs are over 0.2c/impression. Or ~0.15c for an impression via one of those tiny Facebook sidebar ads.
So, after about 3 weeks of this experiment, as a client of the system, you seem to be offering decent value for money
Hopefully it boosts GDNet subscription numbers...