It's easy for me to say that Yggdrasill is perhaps one of the most impressive alternate history RPGs I've read. It's clear that the authors of the book possess two very important qualities in this kind of work: A love and respect for the source, and the imagination necessary to inject a kind of fantasy to make the history become even more vibrant.
The setting of the Scandian kingdoms is given thorough treatment, with lots of call outs to the unique qualities that make a culture interesting. From the call out boxes with notes on lore and history, to giving a chapter dedicated to the culture's religion and gods, the book is as as educational as entertaining (though given that it's still a work of fantasy, I wouldn't use it as a reference for a thesis.)
The system takes a bit of getting used to, but careful reading and a few sample fights should work out all the kinks. Once you get going, everything falls into place. Combat is fast-paced and deadly while retaining a host of tactical decisions that can be used to optimize your chances of felling your opponent. Magic is a highlight of the game given that there are three different magic systems that work beautifully in maintaining the feel of the setting.
If there's one thing I wish the book had more of, it would be the treatment of the other nearby kingdoms. They get a short mention in the setting chapter, but there's not much to work on with regards to what kind of threats they might pose (if any) to the Norsemenn. But that's a very minor issue as the focus of the adventures are grounded completely in the Norsemen's lands.
For a game about vikings, there's more to Yggdrasill than axes and berserkers. There's room for a surprising amount of politicking, and the player characters are encouraged to act like the legends that they wish to be.
The addition of a complete adventure in the back of the book, which includes NPCs, plot hooks and villains all worked out is a cherry on top of the whole thing, helping new GMs who might still have some reservations about getting into a setting-heavy game like this to find their legs and run something that has that Scandian feel.
Overall Yggdrasill is a splendid product, with beautiful artwork and layout, excellent writing. Definitely a must-have for anyone interested in the setting whose ever dreamed of being a viking.
[5 of 5 Stars!]