For what its worth @JNapodano has an estimate of peak sales of 1.5B and DCF fairv value model of 23.25. I don't follow VVUS but I tend to like Jason's reports (the few I've seen anyway).
I think Matt Herper's arguments that the stock is fairly priced include patients don't stay on diet drugs long and no obesity drug has had 1B of sales (I don't know if that is accurate or not).
EDIT: Here is a link to his model (hope he doesn't mind me posting but its on twitter so anyone can see it) http://twitpic.com/8o2er9/full
VVUS—I’ll go out on a limb and forecast that one year post-launch, Qnexa will have less than a 40% Rx share of the combined market for Qnexa and the off-label generic constituents. (Due to branded pricing, Qnexa will have a much higher share of the dollar sales, of course.)