Depends on what market you're in, really - check Ben Edelmann's work and you'll see some pointers but there's too many different ways for tracking to get screwed up to place the blame on one single source.

Best way to pick an affiliate program to promote isn't to look at networks anyway - it's to look at what potential consumers you can access, and what these are likely to buy. If you've got a dieting web site you're probably wasting your ad space if you're promoting hard drives or home remodelling - while diet offers probably won't do well on a computer gaming site. If you're doing PPC, affiliate tracking is actually much less problematical as the goal is to get the sale straight from the click anyway, so you don't need to be as concerned as someone using an SEO/Content/Affiliate pre-sales model.

We thrashed this out in the main forum and some blame the cookie blocking thing on a couple different anti-spyware tools rather than Firefox default behaviour like Dave claims.