I've not used Amazon Warehouse, per se. (Indeed, until seeing your post, I'd never heard of it).
However I have bought, from both Amazon and eBay, several products described as 'open box' or 'factory refurbished' (after, of course, checking the seller's reviews first!).
For example, the Sony wireless headphones I've got were sold as 'open box' but they came in absolutely mint condition. Similarly, I bought a 'factory refurbished' internet radio, which looked brand new when I got it. (I was so pleased that I bought another one as a birthday present for someone). Both of those deals came at around half of the 'new' price.
So there are certainly plenty of bargains to be had by buying such products but, as I've indicated, it's very important to read the seller's reviews before you order!