It's a good expansion, but not an amazing one. Legion sits right there in the middle of the pack for me. It's brought a lot of great new updates and systems to WoW, but also a handful of gimmicky ones, and while the gameplay has advanced in some areas, it's still struggling to recover from the devolution of WoD, particularly in regards to PvE currencies and class design. The addition of time gates to heavily regulate player progression is also one of the most cynical design choices I've seen implemented.
The story is a similarly mixed bag. All of the stuff that isn't to do with the Legion is pretty much fantastic. Suramar might be the best story zone the game's ever had. Unfortunately the Legion are terribly dull antagonists, and the subjection of the player character to a canon lore role via the class hall is just absurd in how poorly it fits into an MMORPG narrative.
The strange thing about Legion is that it seems to swing pretty hard in both directions compared to previous expansions. Most are either generally good or generally bad across the board, while Legion has a lot of absolutely fantastic parts alongside a lot of fantastically awful ones. It's kept me entertained more than WoD or Cataclysm, but I won't be sad to see it go.