This is quite possibly the best Manga series ever created.
All the volumes will have you hooked - and if you're anything like me, you'll end up going through 1 volume (or more!) per day.
Without revealing too much of the plot - it can be best described as an action crime thriller. It has death gods, cops, a small handful of geniuses attempting to work out riddles which keep you on your feet, and many more interesting characters.
It also (quite wittingly) gives you an internal psychological battle, where you try to figure out whether you're with the 'goodies' or the 'baddies', as either side could be good or evil depending on your perspective.
If you're not a huge fan of manga/comics or you're not sure about the series, buy volume 1 first and see how it goes. But if you're a huge manga fan, you may as well buy the entire box set right now!