Please note that I just finished watching the last episode minutes ago and I'm kinda trying to sort my thoughts out writing this.
Darker than Black was never known to be the type of anime that explains everything, many things were and are left in the dark adding a whole layer of mystery to this fantastic world. Darker than Black 2 continues in this fashion but with quite a different approach. While the first season was mostly episodic, consisting of various story arcs with two episodes each, the second season is follows a continues storyline that is however developed quite well in my opinion.
It might seem a little bit cheese and typical in the beginning, but the story soon begins to ravel up and develop a bit of a darker feeling and the missing elements tying up to season 1 are also implemented, without however spellings things out to the viewer. There are always a few plot twists or single episodes that seem quite silly or pointless, but the greater story develops quite well. It is however not resolved completely and will likely continue into a third season in a similar fashion. I wonder whether we can ever hope for a complete unraveling of mystery, though I doubt it.
The characters are very likeably and only partially reoccurring from the first season. The new characters might seem a bit unfitting at first but I like the way they develop. There is also a huge amount of character death, but in a different way compared to its prequel. The music and art continue in the fashion of season 1, the greatest controversy probably being the new opening and ending, though I personally liked them. The change of flavor is noticeable but not off putting.
Also despite being a sequel it will probably also be very watchable for people who haven't seen the first season. The feeling is very similar but the direct links, despite being present, aren't major to the story.
Darker than Black 2's greatest plus is probably it's enjoyment factor. The story, despite being interesting, is still not all that original and often a bit silly, but the atmosphere of the anime just keeps you glued to the back of your seat wanting to know what happens next and most importantly how it will play out. It may not be a historical masterpiece but is definitely worth watching.
So yeah, jump in and watch it. You might even like it if you didn't like the first season to the different approach. I highly suggest giving it a chance.
Overall 8/10.