Robert Solow presents a very clear explication of Piketty's arguments at New Republic. Capital in the 21st Century is, as Solow says, a serious book.
Brad DeLong rounds up the criticism of the book. I have been groping for a way to describe its reception among economists, which is that there are a number of criticisms with parts of it, but that no one I've read feels that it's in any way stupid or misguided. In other words, it is a serious book. As Brad says, everyone agrees with about 70%, disagrees with 10-20% and isn't sure about another 10-20% but can't agree on which parts to agree or disagree with. There are several useful critiques, and several critiques which argue from derp, from which to choose.
Tyler Cowen is less laudatory and more critical, but still recommends reading the book. Suresh Naidu also has an interesting take.
From a cultural perspective, it's weird to see a dense tome on economic distribution and growth reach number one bestseller status. I haven't seen Piketty on any of the national talk shows yet. I dread that day.