As an artist myself, I am a very visual person. The quality of art in a book can make or break the deal for me. I have shelves filled with RPG books that I will probably never use, but were purchaces because they were nicely illustrated. I was initially resistant to Eberron, for example, but all of the art by Wayne Reynolds and Steve Prescott really sold me on it. Good art, of course, is subjective; some people like one style of art, while others think it is garbage. Rather than try to appeal to the broadest audience possible, the better route (IMHO) would be to make the art in your book consistent. It drives me crazy when there are some really well-crafted and gorgeous illustrations in a book, and then you turn the page and there's some dreck that looks like it was done by some half-rate hack at the last minute. I would rather have a book whose artwork is consistently mediocre than one which jumps from awesome to crap and back to awesome. It's jarring, and it doesn't help tie the theme of the book together.