Although I've never used MDF beadboard, I have used MDF in other applications and I've been very happy with it. The pre-paint is usually just primer and may need to be painted. If that's the same in your case, definelty paint it.

MDF paneling works well as long as it doesn't get chipped or scratches since it's hard to repair the surfaces unlike wood products.

I used this product to cover ceilings in two bedrooms and a bath that had water stains and light damage, and I did wainscot with it in the bath. This was in a rental home, but it added a great cottage look and did the trick.

Do your attic walls already have drywall on them? If it is the bead board that i have in mind, it will not be sufficient without backer. It will warp pretty badly. If you do choose to use it without backing I would highly recommend painting the unfinished backside as well to try and prevent warping.