I can second Psion's recommendation of Grimm. Incredible little book and fairly priced, too. If I had more money, I'd certainly be checking out more of the Horizon line.
What he said, and others, Sidewinder: Recoild, most recently.
I've also bought tons of books based on discussions and reviews on these boards. I'd say 95% of what I have bought is mostly because something I read on ENWorld got me interested, and led directly to a purchase.
Of course, I'm buying much less now, as most of what I have bought has not been of much use at my table. However, ENWorld is still driving my purchases to a large degree, as I still rely on the discussions here to help choose.
More often than not I've purchased a product that ends up winning an ENnie because of the buzz for the product before-hand, rather than after.
ENnie winners definitely get a second look after the awards. Though to be honest, I'm more likely to buy a product whose publisher is a friendly member of this community who posts helpful information than I am just because they win awards.
I don't buy a book, play a game, or see a movie because it won (or was nominted for) an award. But the nomination or award will give me reason to consider a product I had previously passed on, or may draw my attention to a product of which I was not aware.
Winning the ENnies was an important part of securing world-wide distribution. It, and Monte's great review and the guys at Games Plus and Blackhawk, got us distribution. Although that's obviously an abberation, that's quite an accomplishment.
I know a lot of people gave us a chance because of it.
I will probably take a harder look at a product that won an Ennie; I know I will with Grimm and Grim Tales. Usually though I already own the stuff up for the award so - with me at least - there is no 'after Ennie sales spike'.
Actually, yes I have. If there is a book/publication I was already "thinking about" the Ennie _nomination_ or winning would be the thing that pushes me to think it's worth it to get it.
I never buy things just because they won the Ennies. I buy them because I know the product has some info I think I can use, the product fits within my free amount of cash, and because the reviews, and/or my own experience with that publisher's work indicates that the quality is good.
Frankly, due to the fact that I live in Israel and few game products get here (compared to the vast amount of stuff out there), my selection is quite thin, so I usually buy PDFs, and if I bought a PDF froma publisher and I did not like the quality I will never buy from that publisher again.
In short, publishers should pay attention to the quality of the products they publish. If the product is good it will sell. Yes, the Ennies might affect some people's opinion on your products, but if the quality is not up to their standards you ed up, and they will be much less likely to buy your products again.