This is one question I have always thought about. In a ecommerce shopping cart when do you have a 'buy now' or 'add to basket' button next to a product on a category page?
This is assuming you have individual pages with further product information. Many big retailers have both a 'add to basket' and 'view more info' button, however I rarely believe someone would make a purchase without first going to the product page.
Would be really interested to hear people's thoughts here.
I'd avoid it, but only usability studies can tell you whether or not it's worthwhile.
It depends completely on who your user is and their current situation. If I came to your site looking specifically for Acme Product ABC-1 and I already know all there is to know about it, I'd probably click the buy now button on the category page. If I'm just browsing your site, it's a lot less likely.
If you look at newegg.com they have the buttons on the category pages, but they also tell you quite a bit about the product on that page. Target has an add to cart button in its quick info popup, but I suppose you could argue that's not quite the category page.
It probably depends on the type of product. As you say, if you already know about the product, or order on a regular basis, it can speed up the order process.
If most of your products have options (sizes/colours etc) you could argue that it's pointless because you'd usually have to go to the product detail page to select your options. But there again, you could only add "add to cart" buttons for products that didn't have options?