Got my MMIV today. I really like the art so far. Any of the pieces by WAR, Anne Stokes, Arnie Swekel, James Zhang, and Daarken are really fantastic. Actually, 90% of the illustrations are really good, only Wayne England is a little below par, but he's never been my favorite.
It's great seeing Arnie Swekel and Raven Mimura back doing D&D art alongside the excellent 'newcomers' James Zhang and Daarken. It's a really pretty book.
The dragonspawn are really varied and, IMO, the Bluespawn Godslayer, Greenspawn Sneak, and the blackspawn raider are the most 'boring' ones in the set. The rest have some unique abilities to help differentiate them, but nothing overly complex.
The statted up NPC monsters are a really mixed bag. The yuan-ti are pretty interesting, but that's about the best I can say. They look pretty utilitarian ie useful, but nothing else to write home about. The lizardfolk get the most love, with seven pages! The rest get about 4-6 pages each. The Yuan-ti get the least.
The Lolth-Touched template is pretty lame. Almost not even worthy of being a template. The web mummy, otoh, is pretty cool, though it almost looks like the idea was lifted from the Tome of Horrors II. Just for the sake of comparison, I like the Corpsespinner in the ToHII much more than the tomb spider, but I like the Web Mummy better than the Corpsespun template. Weird.
On a quick skim, my favorite monsters so far are the Vitreous Drinker (an undead created by Vecna that steals a creature's sight and can look through its eyes), the Defacer, the Avatars of Elemental Evil, the Balhanoth (a mage's worst nightmare!), the Concordant Killer, and the bloodfire ooze.