I looked at a lot of other Affiliate apps and in the end, they all promised alot, looked professional, but didn't deliver when it came to technical help, customizations, features or support.
An important part of our selection process was that their solution offers the ability to implement 3 tiered (Rep, sub-rep, customer) commission structure - which other apps can't support.
SalesRep by Chief Software may not look fancy, but is very functional, they are SUPER responsive and their support is excellent. They provided many look and feel customizations for me for a very nominal fee.
I am happy to be working with them and recommend them to others.