I got a free month at some point. Yes, the previews are longer. OF pages with a monthly subscription can't lock wall posts, so everything on his wall has a tip goal that says "if you tip $x I'll send the video to your inbox. Once the tip goal is reached, I'm increasing the price by $10." The tip goal is usually 10 sales worth and he still sends unlocks through dms.
It creates artificial scarcity and encourages gooners to panic buy content. If you pay into the tip goal, don't do it near the end of your subscription. He still has to manually send the video to you, which could take hours or days.