I wasn't impressed with this. I really wanted it to kick a$$. It didn't. About the only thing that impressed me was Mongoose actually listed/used all the Cimmerian deities mentioned by REH.
That was cool (though their interpretation wasn't, IMO).
It could be better, there's no doubt. I as a bit disappointed when I first opened the covers, but I've since found value in it. I've been using it as a background for my Cimmeria-based campaign. There are some good ideas in it and many I think you would bristle at because they're non-canonical (like the mention of the Titans). There is a lot about Cimmeria that could have been grown in the RPG universe using this book. That defintitely doesn't happen.
The major strength of the book, though, is its largest section. Several large clans are described in detail. The author (Lawrence Whitaker) did a good job of making each clan unique. Some favor your normal Conan-esque type Cimmerians, but other clans have their own "feel" that is different from the clan Conan came from. For a game set in Cimmeria, as mine is, that's gold. It helps me establish the different Cimmerian clan-cultures that the PCs meet during their travels.
An energetic GM could even take these Clan descriptions and make sub-races, with stat and/or skill modifiers, for the game. These would be adjusted Barbarian templets with maybe some other classes thrown into the mix (Borderer, Nomad).
For example, those of the Blue Fox clan get an additional +1 STR at character creation due to the fresh water and minerals that permiate their grown foods and living areas. But, these Cimmerians are highly superstitious, so they must also suffer an additional -1 INT penalty (for a total of +3 and -3). Another clan is good at leatherworking, so it's a requirement that they have at least one skill point in Craft Leatherworking and maybe they get a bonus to Animal Handling and Craft Trapping.
You get the idea.
All-in-all, I'd say that Cimmeria isn't what I wanted it to be, but it's not a total waste. There are some good ideas in the book (and some not so good ideas--why would you spend precious pages on Cimmerian weddings???? When we could be looking at weapons specifically and uniquely used in Cimmera, like the Cimmerian Dirk that is found in another supplement. Or, how about replacing the wedding stuff with Smithing rules for Cimmerian steel?), and it's a useful supplement for a game that is going to spend some time in Cimmeria.
I'd give it a "C".