For me, usage has been a bust. Full rights is where I've had my most success. Sure, if you have previously published stuff, put it up for usage and see what happens. But from my experience, the best way to make money here is with full rights.
At first, I had visions of articles selling over and over again for usage but in reality only a few (out of hundreds) have ended up doing that. Even so, those few have just barely earned out past the full rights price. When they do, granted, it is awesome! And I see the ultimate potential there.... but my successes have come by full rights purchases.
The best way to get full rights is to price usage and unique higher than you might normally consider. I shudder when I see articles going for $5 for usage. My head spins and all that good stuff.
Anyhow, that's my 2 pennies . . .