Most are just scams. Be prepared, with the holidays coming, many bloggers get freebies to gift. If you look at prior giveaways or reviews that a blog has done you can get a feel for their following. Often, it is a few other fellow bloggers who have entered to win, or commented on a blog post featuring an item. I did send a few small items when I first started selling here, but quickly realized that it was not worth my effort.
That said, if I were approached by a hugely successful blog (like Oh Happy Day!) I would respond in an instant!