Always, always always presell a visitor before sending them on. Especially in the case you lay out (where the sales page isn't great), you need to warm your visitor up to the product.
People love to see your own personal results and hear your own personal success story with a product. It increases conversions ten-fold (in my experience).
It's natural for a visitor to distrust a product vendor. After all, they're in it for the money and would likely say anything to get a sale -- or at least that's how most visitors feel when they land on a sales page.
You remove that distrust by demonstrating success from a "neutral" third party. You're not really a neutral third party since you're an affiliate, but again, that's how the visitor will see it.
So always pre-sell first. It's best if you can get them on a mailing list and do a series of follow-ups to further presell before offering the actual product, but even just a one-page review demonstrating proof is far better than a cold referral.
In fact, I think I'll let everyone who reads this answer download a free copy of my Preselling Secrets report (from PresellingSecrets.com). It shows how I made $20,581.92 from one affiliate promotion of a $77 product. It's from a few years ago, but the principles in the report are timeless.
Here you go: