Most of the time it's a p.i.t.a. to make any serious money with them. And they're very strict... if you're aggressive you'll get your account reviewed/suspended at some point. That's when they're the worst. It can take weeks to talk to someone and months to resolve.
If your plan is to send shady traffic... don't. They don't like it and they won't pay you. Plus, they take so freaking long to pay out that they're highly likely to sniff you out before the check comes due.
If you have a good plan (and usually loads of traffic) then you can definitely make it work, though. You move up the commission scale pretty quickly and can get to 8.5% with relative ease.
Don't bother pimping low-end stuff, though... it's not worth it for the nickels and dimes you'll make selling books and such.
My best month I made just a shade over $46,000 with them, so it's definitely possible to make good money as an associate. But most associates don't even come close to making anything serious (like just about every other affiliate program).
And they're famous for slapping affiliates that rake it with them. They made me agree to a new affiliate agreement once I starting making good money every month. They basically said they would pay me for everything except the item I was promoting. Go figure.
Strange way to do business... but they definitely don't think they need affiliates at this point (imo). And they treat you that way. Any big associate I know has at least one horror story working with them.
So it's kind of strange... in some ways they're the most affiliate friendly site around because of their product selection and how easy they make it to get started. On the other hand, they're far and away the worst I've ever dealt with at the higher end.
Get creative, though, and you can do well.
One last thing... I don't think I've seen a slower-paying program than Amazon. Takes months to get paid. It's a hard way to go if you're paying for your traffic. You've got to be able to float it for an unusually long time. Could be expensive.
Oh (last thing... for real), and they're tracking is the worst. Virtually non-existent, really. So it can be difficult to optimize paid campaigns. As in virtually impossible.
Good luck.