I believe Yggdrasill is good attempt at capturing role-playing in the Norse or viking culture and mythology. The world overview is sufficient for a start but serious GMs will complement this with the other world books. My main issue with the setting is that it tries too much to stand on two feet. The authors have chosen to portrait a mix of both the historical and mythological Scandinavia and while the goal may be to allow players to emphasize the version they like best, I would have preferred if they had chosen either one and concentrated on that. Personally I either want to play in a gritty historical setting or in a high mythos viking saga, no a luke warm mixture.
I find the rules easy to learn and well suited for the setting. I especially like the fate system and believe they could have included more similar mechanics regarding fame, sagas, an other typical trappings of the Norse lore. When you chose a defined setting such as the viking one, I think it should make an even greater impact on the rules. The magic system deserves special mention for being well conceived and thematic.
Overall I think this is a nice product with some interesting ideas that unfortunately is somewhat bland.
(my views may be somewhat colored by the fact that I have been raised on Norse sagas and mythology)