A recent conversation on one of my many Yahoo Groups/ListServes involved illustrators.
The debate was over using a college art student and getting the work done cheap or paying a professional and getting it done right.
I think it depends a lot on the illustrator! Art students who are responsible and looking to break into the field will be eager to do their best. I have seen some really great illustrations from amatuers.
Then again, going with a know author can lift the quality of one's book, too - just be prepared to pay the price for it!
Either way can work well for the author. In my case, I have an artist who is a professional but not done a lot of work in the field. He has improved tremendously over time and I find the less input I have into the finished cover art, the better!
I write, publish & promote - think I'll stick to that!