I've used the DoI twice. For 2 years around 2005 and for the past 3 years. I decided to stop advertising there because I have not gotten much work at all from them. They recommend advertising for 3 years in a row to establish a presence and get steady work. That hasn't worked for me. It's expensive and I don't think it really pays for itself.
They actively encourage you to promote your work via their site and social media (updating your images, writing blog posts, etc..) as well as through your own site and promotions (upon signing up you get a discount to the Association of Illustrators site and a mailing list service). I figure why not save $2500 and utilize those other methods?
Another thing that I don't think gets mentioned enough in discussions I've seen about the DoI is the timing of their reservation, art submission, and book release. Granted, you do get access to their site to promote images as soon as you sign up and I am aware of the long lead time that producing and distributing a printed book requires. Still, once you sign up, they will try to get you to reserve a spot in the next year's DoI before the first one is even out.
Now the DoI does work for some people, but I think your advertising budget is better spent elsewhere.