I'm very happy to return to the world of Mistborn, especially that of the first era. That said, this book largely fills in details between events that you already knew, or had guessed at. Even in the author notes, Sanderson acknowledges that this book could have read like a set of footnotes. For me, it allows us to reconnect with loved characters while trudging along a narrative that is just okay... There are tantalizing hints behind the further secrets of Mistborn, writing out that which could be analyzed from the Wax and Wayne novels.

But who am I kidding - if you're thinking about reading Mistborn 3.5, there's no way to keep you from reading it. I personally think the spoilers are sufficient that you should wait until you've finished Bands of Mourning (Mistborn 6).

I spent most of the Wax and Wayne trilogy not believing in Kelsier's survival, and believe that spoiler is worth hiding. I also think there's a lot of stuff about how Era 2 develops (Spook being the Lord Mistborn, for example) that is instantly spoiled. Those things are fun, because I thought Elend somehow survived. On the bright side, the whole Cognitive realm, the introduction of the Ire, the various characters like the Wanderer... these were really interesting and worth the investment of time. That and seeing Ati / Ruin and Vax / Preservation at their finest... If there's any complaint, it's that the novel doesn't stand on its own and I see no reason why anyone would re-read. But I love Kelsier, Vin, Elend etc. too much to give anything less but 4 stars for a return to their world.