There's only one site out there I would join today.
It isn't any of the big mega-sites... you kidding me? They've all got the same girls on the tours, which says "we've all got the same stuff inside". How many ways can you take that Matrix set of Skye in the blue outfit or Aria in the red patent leather and make a tour out of them? Surf around, you'll find out.
Trial? Don't make me laugh. I don't trust it - and that was BEFORE I became an adult webmaster. Tell me I'll pay $X for a yearly subscription or even a 6 month subscription, then you've got my attention. That tells me a few things:
1. The site plans to be around for a while
2. I'm getting some value for my money
Besides, from a webmaster's point of view - surfers are far more likely to forget about a bill that recurs in 6 months or 12 months than one that pops up on their statement every month.
Monthly fee? Hell no... I've got enough monthly bills. The minute you tell me it's $X per MONTH you've changed my line of thought from "damn I'm horny" to "oh shit, what bills do I have left this month? I'd better check the online bank statement..." and I'm gone.
Offer surfers a 3/6/12 month membership and you'll see wallets getting pulled out. It's what the customer *perceives*, not necessarily what you tell him or show him. Make him think he's getting a real deal and he'll hop on it. A trial membership or $1 membership doesn't say "deal", it says "you're gonna get burned on this, don't trust it".
Then again, I'm a spendthrift who gets horny, rather than a guy who's always horny and thinks with his little head before his big head.
Oh yeah, the only site I'd join? Nina's. Because it's so damn different and it SCREAMS personal attention and interaction. This isn't just some site where you can jerk off to the girl, this is a site where you can get to KNOW her by interacting with her on a regular basis. A lot of amateur sites have this angle, but they don't push it; instead they try to push themselves off as "just like the megasites but with a personal touch".
Just my

...I've not been in this so long that I can't see things from the surfer's point of view.
All of you guys arguing... it's a moot point. Different traffic converts and retains differently. People in different niches convert and retain differently. You can make a generalization about retention rates etc. but arguing over it is pointless unless you can take over your arguing opponent's sites and traffic and see things/change things for yourself.