AffiliateWP works pretty well out of the box with minimal configurations necessary. We needed a professional looking affiliate system for a WordPress site, and AffiliateWP covers most of our needs. Works well with both WooCommerce and Easy Digital Downloads, and the generated markup for frontend elements is clean and easy for a developer to customize or integrate with an existing theme.
The "Creatives" section added through the plugin functionality leaves a lot to be desired. Doesn't seem fair to complain about this considering the plethora of other features the plugin has but this area seems to have the most obvious room for improvement. Being able to provide more to affiliate users in terms of helping them sell products or having more ready to go ads with affiliate links would be nice to see.
If you need an affiliate marketing program and already have a WordPress site, you should try AffiliateWP!
Not having to have a custom tracking system developed and implemented was good enough alone, but AffiliateWP provides additional features that make the actual affiliates appreciate it, too. Providing immediate updates regarding tracking, referrals, commissions, etc, puts a lot of people at ease. The cost savings with using the plugin compared to custom building something similar is a huge benefit.