There are not enough good things I can say about this book. It's my personal favorite. Everyone I've leant it to has enjoyed it. Not everyone liked it as much as myself, but they still all thought it was good.
It's a full story with a beginning and end. No "to be continued", no loose ends. The artwork is phenomenal, and changes up with the story as needed. (My favorite is the X-Men & Sentry chapter, which appears to be all done in watercolor. This changing artwork style will make a lot more sense when you read it, so just trust me, it works).
All I will comment on regarding the story is that The Sentry is Superman in the Marvel universe. Superman helped grow the comic industry, and helped pave the way even for Marvel. This work is in part a tribute to that, at the same time exploring what kind of nemesis such an all powerful person could possibly have.
If you liked Paul Jenkins and Jae Lee's collaboration on The Inhumans, then this is a no-brainer purchase.
It's so good in fact, I think I'm going to re-read The Sentry right now.