Deep Dark Blue was very timely for me, as it came just as I was planning a submarine-based campaign. Its mechanics for building characters (and campaign) around the ship and crew was a great way of getting players into the mindset of being a crew rather than just a party.
I also appreciated the way in which crew cohesion is designed to be a mechanically significant factor in the success of the ship during missions; it both gives the players incentive to role-play as a group, and gives the GM a focal point for telling a good, character-driven story.
Like most Evil Hat products, Deep Dark Blue is far more focused on setting than mechanics. The "crunchy" elements of the manual make up a small portion of the whole, and I appreciate the depth of material that their work offers for inspiring the GM as a writer.If you are looking for a major expansion to the game's mechanics, however, you may be disappointed.
If I have one gripe about Deep Dark Blue, it is that they have a huge number of characters that break old gender, sexualiy, or cultural norms. I appreciate their choice to be inclusive in principal, but it comes off as overdone and fetishistic - it appears to be more of a cynical marketing ploy or tokenism than inclusion.
All-in-all, a great module / expansion, well worth the $4 I ponied up.
[4 of 5 Stars!]