I completely agree with ccharles90. I think a lot of marketers don't respect their lists and assault them with sales messages.
Email marketing is really a way to have an ongoing relationship with your list and build an opportunity for them to know, like and trust you enough to not only receive your information, but to purchase and refer. The sites that I'm spending the most money on right now offer me great value even if I'm not in my buying mode yet, but they have received my dollars in the end because they respected me as a client and not a target. I'm not just anyone to them I'm a valued individual and I can tell you right now I have spent major money and more importantly I promote them to others.
So in my opinion, if you want to market through email marketing you will see more bang for your buck if it isn't a sales assault but a relationship builder.