From an affiliates point of view I absolutely love the way NATS handles stats and campaigns. My big beef with NATS is the encoded URLs (different ref code for every site), but I believe there is an option to allow the ref codes to be unencoded (and the fact that most of my sponsors on NATS choose not to enable it really chaps my ass). The way it handles promo material is a real pain as well.
Those are my pros and cons
