I doubt too many people, even on EN World, have bought books solely because they won an Ennie, sight unseen. But it definitely is a contributing factor, at least for me. I view the Ennies as a strong mark of quality, and at the very least it can tip the scales when deciding between two competing products.
Personally, I buy very few books, but I did wind up buying some of last year's winners for GM day, including MMS:WE. I have also recommended various Ennie winners to other people that I thought would be interested in a specific topic.
If I had some more time and money to spend on buying RPG books, here's a few I would *definitely* get: Midnight, M&M, Redhurst, Roleplaying Tips Encyclopedia. Some of these (like Grimm) I had never heard of before the Ennies.