from the stand-point of an 'end-user', it's easy to get enough to jerk to from just browsing free content. Point of fact, even though I now have access to a ton of content through work, I still do the 'free spank' approach half the time.

The times I have bought a membership somewhere (which I do about two to three times a year) it is usually something where the free pics or content has a gap I want to take a look at. They have a still of an action about to take place, and then another of the models after it has happened, but done in such a way that I HAVE TO SEE IT!. A good example is a facial. If there's a pic of the happy recipient about to get it, and one of the 'aftermath' and they're done well enough, I have to get that membership to see it actually happen.
Don't know if that makes sense, not exactly at my most eloquent today (got a head cold and I'm in the middle of a last minute move on friggin' New Year's Eve!)