I know Neil Gaiman took some flack when Norse Mythology was first released mainly because he wasn't so much making new mythology for the Aesir and Vanir as much as he was retelling his favorite tales from the Eddas, but honestly it's kind of nice to have an introductory text to the myths, since there really aren't that many accessible texts introducing readers to the stories. And given this one is written in a PG tone, it would probably help children reading/watching Marvel's Thor get a better grip on the source material.
That being said, I'm not a huge fan of Norse mythology, so it's taken me some time to delve into Gaiman's book.
The first thing to note is that this isn't a complete retelling of the cycle. He glazes over Odin hanging on the world tree and Odin's eye in the Well of Mimir. Many of the Gods who get brief mention in the more complete sources aren't even mentioned in here.Which is fine, since Norse mythology doesn't lend itself well to Greek style retellings, where you can isolate a particular God for one story or two that illustrates who they are and how they function in the pantheon.
Was there information in here I wasn't overly familiar with? Yes, more than a few of the stories were one I had not heard, or had not heard quite as much of as what's in here. (Freya's marriage to the ice giant, for one; Thor drinking 2/3s of the ocean another. Was also nice to get a fuller story on how Tyr's hand wound up in Fenrir's stomach.)
Ultimately, I ended up enjoying it. As much as I loathe the Norse, Gaiman does a good job of making an engaging narrative out of the stories he decided to retell. And it would be a fabulous resource for folks looking for something that doesn't cure insomnia like the Eddas.