Affiliates

Has anyone experienced the effectiveness of an affiliate system for product sales?
If so, which service to choose from then?
Is it crucial to the quality of the sales system? Also, how should I recognize if it is worth to develop own simple sales system (referral link> account crediting > statistics (conversion rate, number of downloads, purchase rate)> payment)?
Please mention if there any catalogs of affiliate systems to submit our own?
How effective are they?
Thanks to all

See original post by kirill79