I've used
Sexy Ads . Net since the beginning .... the retention is awesome - the members customer support is extremely personalized. They have awesome advertising materials.
The people that run it are a couple that met through personals and they are very very hands on. They also have a gay version of it.
They also offer niched tours to target your traffic better.
If a member signs up for a free membership and later turns it into a paid - you get credit.
Can you tell I've made a LOT of money with them over the years
And .. it's NOT oversaturataed in the marketplace, although I believe they have something like nearly 1,000,000 active members.