It has the most variety (love the puzzles!), but Gul'Dan's decks themselves were pretty bland to my taste. No crazy spells, powerful demons or flashy combos, mostly pretty straightforward Zoolock / Demonlock.
It was a great addition with the puzzles, those were fun. In general, I feel like people have been too harsh in their judgement when it came to the Book of ... content. I like that the Hearthstone team is experimenting with narrative mechanics. Hearthstone isn't exactly an ideal platform for storytelling, but IMO they're actually doing a great job of making it work. I for one am very invested in the Book of Mercenaries story line!
Not only are the fights pretty good, it is awesome to see gul'dan picture change with each fight as he gets more evil.
They've all had changing hero portraits, and I agree, it is a nice detail. It's also nice that they've taken the time to give existing cards new, more appropriate art (for storytelling proposes that is). In general, it seems a lot of time goes into the details, and that's something i personally appreciate.