I've only ever used ClickBank. It is reliable, pays well (depending on the products obvs), and is very easy to use as it is free and all you need to do is sign up and hook up your payment details
my only criticism is their are not a lot of "great" products to sell in some niches
weight loss, fitness, and finance are niches with a lot of great products therefore their easy to sell, however, I work in the music niche, and the products just aren't that great with very 'scammy' and cheap looking landing pages which just don't convert that well.

I only use clickbank now and I am happy with their paypal payment. But they should better monitor the quality of their offerings because some products are not the best and look suspicious.

Clickbank is a great place for beginners to start. Their offers vary, and depending on one’s niche, someone might have a problem generating traffic to that affiliate page. Clickbank does show gravity but it doesn’t show profitable that product is so I use an external website to see if its worth promoting as there are many products with low competition but high payout. Support is fantastic with new webinars being added. Pay out is good depending on the product you choose.

Best - Clickbank, good support, payout on time and very reliable. Quality offers as well.

Now I work with clickbank and I am satisfied with the result. I think you have to be a professional to work with them because not everyone can find a good product in the trash. But if you succeed, these guys can pay well, that's all I expect.

ClickBank is a valuable tool for affiliates, but it could be a little more affordable as far as trainings are concerned. Setting up pages that will earn you an income, and that will also attract others to want to promote what you are selling is a challenge until the learning curve is conquered. With those issues out of the way, it's a way to earn money, and to have others promote your wares while everyone benefits. ClickBank is a valuable service with a tried and true record.