To Ad, or not to Ad, that is the question

I've been approached by two companies this week wanting to advertise on my blog. Hmmm, confused I am. Can't really tell if it is random spam they are sending or if there really is something on this site that's attracting millions of potential customers for someone. (Well, I doubt it's millions because I have logs of server load and visitors).

Should I sell some space? Should just say no and keep this ad free ? Ah decisions decisions.