This DVD set is just the individual season sets packaged together. So, if you purchased the seasons separately, you are not getting anything different besides a box to hold all the individual season cases. All of the extras (and there are a lot of them) are exactly the same, and nothing (at least in the DVD version) was added. I cannot say what they did with the Blu-Ray complete series set, although from what I have read, it has the same extras as the DVD set. The extras include audio commentaries on many episodes, which rotate various cast members, crew members, and writing staff. There are many commentary tracks in the early seasons and fewer as the seasons continue. Then there are many deleted scenes (the best of which involve Creed and "Schrute Bucks," blooper reels, webisodes, promo material, and more. If you love watching bonus content, there are hours worth (apart from the commentary tracks) to keep you busy.
Overall, the series is very good. Like many shows that last nine seasons, it was not as good at the end as it was in the beginning, especially after Steve Carell left. It had a large ensemble cast that included Rainn Wilson, Jenna Fischer, Mindy Kaling, John Krasinski (who, other than Carell, has probably had the biggest post-office career), and Craig Robinson. Many very recognizable comedy actors and actresses also appeared in the series in both large and small roles. The show definitely lives up to the hype, and even though the last couple of seasons are not as good as the early seasons, they are not as bad as some people make them out to be. So, if you are a show fan, this is a great pickup.