Hit or miss for me.
PHB: Mostly ok. Preferred a lot of what was in the previews to what's actually in the book. I also art done with paint brushes rather than the brush tool, so while I like most of the 4e art over the 3e art (most of which was dull and boring), I still like a lot of the 2e stuff best. My biggest disappointment is that they recycled the equipment pics instead of doing new ones.
MM: Wow! Now
that's some good art! Forget the recycled stuff (of which there really isn't very much). Most of the new art is really fantastic! I'm sure most, if not all, of it was done on the computer, but it's not as easy to tell as a lot of what's in the 4e PHB.
DMG: I skimmed this one very quickly but I was
very happy to see that the illustration of the elf warlock doing a ritual in the woods had not only made it into the core books but had also made it in as a 1.5-page section header! Woot! That has got to be my most favorite 4e illustration so far.
As a budding graphic designer, I really dig the layout. The pages are nice and clean and easy to read while the photoshopped stuff adds a nice flourish without being over the top. There's nothing that really impacts on the readability of it. The use of lots of negative space also has the side effect of making the books more environmentally friendly and therefore sustainable as they require less ink to print than the 3e books needed (I asked the Rouse if this had been a consideration but he didn't think so - just a happy consequence of the design direction!).
On a side note: Has anyone else noticed that of the five 1.5-page section illustrations (which is half of the total) featuring tieflings, in
every instance the tiefling is all the way over on the left-hand side of the illustration, thus making it the first creature viewed in the illustration and often the only creature on the left-hand page. Coincidence or conspiracy? You decide ...

EDIT: It's not just the PHB. The one in the MM has the tiefling on the left as well. Although I will admit that the section illustrations in the DMG that feature tieflings place them over in the larger right-hand portion of the image.
Side note 2: It appears that Regdar and Lidda, at least, are still around (if only in the art -- I haven't read all the text so I don't know if they're mentioned by name).