Free is almost always better.
I think in some cases it would be worth it though.
It's good to catch a surfer at the search engine for certain phrases. If they punch in 'free pics of aria giovanni' there's a good chance they haven't bookmarked a bunch of LL sites yet.
The only problem is that when someone does punch in that phrase, the natural listings will usually sufice.
If you can write an Ad that is descriptive enough you will get some traffic. If you get traffic you will get some people to bookmark your site.
You only pay what you want. If you're curious give it a go. Google AdWords is
much quicker to set up than Overture.
They will also show an ad live then eventually get around to reviewing them.
Give it a whirl. It can be a good way to jump start a site.
My .02