1. Read him.
2. He is surprisingly sloppy on several points - most fantastically the tax history and the minimum wage history. Magness and Murphy provide a good overview of all this in pages 1 through 10 or so.
3. Stop reading Magness and Murphy at about page 10.
4. Keep in mind that Piketty is not some random lefty. He is at the top of the field in work on wealth inequality, long run inequality trends, etc. All the other top people in the field have co-authored with him and use his work. They trust him. So when Phil Magness - not in the field at all - tells you he can't figure out what Piketty is doing from the technical appendix your reaction ought to be "well this sounds right - no one would expect Phil Magness to really understand everything Piketty is doing from the appendix - this does not shift my priors at all about Piketty because I have been given no additional or surprising information."
5. Keep in mind my point 2. This should shift your priors about Piketty. But you have two options for forming a forceful opinion on the inequality: (1.) replicate his work yourself to understand what and why he did what he did, or (2.) wait for someone that knows this material better than you to do the same. Only then should you think anything like the inflammatory attacks that Magness and Murphy have leveled.
6. Until either option in my point 5 has transpired, get a broad sense of the literature and keep an open mind. Know where there is more disagreement (post-1980) and where there is less (1900-1980).