I'm about to launch pre-registration and then limited release for a new web app and am looking at using viral marketing apps like Kickofflabs, Referralhero, etc.

The goal is to get people to sign up and then refer friends (and others), incentivizing them with discounts and free services.

Has anyone here used any of these services? Could you share your experiences and whether there are any you recommend or don't recommend?

I'd really appreciate any comments / feedback / advice.