I've played most of the AC games, many of them multiple times, and was really looking forward to the release of Mirage - especially after being disappointed in Vahalla. Sadly, it was such a let-down.
While I loved the setting of the game, the storyline is short and boring compared to others in the series. Side-quests are repetitive and mostly inconsequential, with little or no opportunity to deviate from the main storyline and get lost in the world/ map for a few hours. There is extremely limited weapon, outfit, and gameplay customization, leaving the game feeling quite forced and rigid.
Given I could easily sink 150+ hours into the other games, the price point on Mirage just didn't feel worth it. You don't get much bang for your buck and are left wanting at the end, wondering where the rest of your money went.
Forever an optimist, I'm hoping this (and to a lesser degree, Vahalla) was just a blip in an otherwise excellent series, and that Shadows brings back the AC spark.