Sekirei's strength as a series lies in its ability to combine the shallow trappings of action and fanservice with a strong character dynamic and surprising emotional depth, culminating in a show that's fun to watch and sucks the viewer into the story. The second season, Sekirei: Pure Engagement, expands on what the first season did well and does it better. Now that the premise has been established and the main cast is fully assembled, the series is given free reign to expand on the scenario and develop the characters. What results is a season with a tighter plot than the first, with a satisfying blend of humour and drama, and improvements all across the board.
Since it picks up where the first season left off, it's only prudent to watch it before jumping into season two. This Limited Edition DVD/Blu-Ray set comes with a special display box specifically so those who already own Season One can store it and Season Two together. It also comes with an OVA that takes place betweeen the two seasons, which bizarrely is on the 2nd Blu-Ray/3rd DVD. It would have made more sense to put it on the first disc for both. There are also audio commentaries on episode 5 and 10 by the director and voice actors.
The audio commentary for episode 5 officially confirms a third season is on the way. I'm looking foreward to it!