Me and a few partners are starting a small affiliate network this year. We have been in contact with DirectTrack for a while.
The system is very solid. I have tried the demo... a few times, and I like what it does. But, the software is only the bones of an affiliate network.. keep that in mind.
You can choose from 3 versions... Merchant Standard, Merchant Pro, and Network. The biggest difference between them is the number of domains you can run offers to. If you are building an affiliate network... the Network Version is a must. There is more to it, so check out the website.
The startup is a little pricy, but... just shows how dedicated you are.