Overall, I love what they've done to sleek down the basic rules engine, and character design and alignment have a "back to basics" feel that I admire. I worry that, with just the basic rules, there's not enough options to keep characters from looking pretty similer, once a group has worked out what the best combinations are, but that's a minor issue.
My biggest gripe--they've left out a lot of magic items, they've left out a lot of signiture monsters, they've left out a lot of main classes, and in each case, there's a "but they'll be coming up in future books!" I feel a bit like I spent more money for a lot less content--the fonts are bigger, the pictures take up more space, and they've just told me that I'll be able to purchase what I feel like I'm missing in my core book. So, they've clearly got their release schedule filled out for the next few years.
Illustrations...meh. Not as good as 3.5, definately not as good as 3.0.
These books are great on the mechanics front, but I'm finding them very dull to read. I'm not sure I can sustain interest to spend the $$$ on a full library, though I'm sure I'll get the Book of Fighters and Wizards when they come out.