We use them for some of our key landing pages, but for the majority of our pages (product pages, really) we let Google decide what to use for the meta description. These pages get a lot of medium to long-tail traffic and I feel like Google is going to do a better job of inserting bolded, keyword rich text from the page content and generating a better CTR than if I were to plug something in there myself.
Would I be better off optimizing the the description for more of a direct match keyword instead of doing what I'm doing now (using Google generated descriptions to optimize for medium to long-tail keywords)?