Just picked it up and have only had a chance to flip through it as I am at work right now. First off, the art is pretty good. There is some stuff that I don't recognize (although some certainly is recycled) and some are really, really good.
As far as the contents are concerned I've got mixed emotions. Some of the stuff is nice that they have added to the book and it is convenient that it is all in one location. Having said that, there is a lot that they left out. For example, I think hiding could use a better explanation (and in fact was completely left out of the book) as I have played in several groups and each group handles hiding differently. Another example is that they have a nice section on grappling and they discuss constrict and other abilities like that but no where do they talk about the attach ability that some creatures possess. This would have been really nice. Finally, the thing I think I hate the most is that they used many of the same grid and mini pictures from the PHB for things like flanking, charging and how a line works when casting a spell. I think the book would have been so much better if they used new pictures to illustrate some different examples. Many of these things are still played differently from table to table in my humble opinion.
In a nutshell, the book seems good and handy but the book fell short of resolving some of the rules questions that I have seen show up which really, really disappoints me. In fact, WotC called this a celebration of the rules but for the most part it felt like a regurgitation to me. Some stuff was new but not as much as I had hoped for or expected.
Hope that helps!