This volume of short stories and longer novella-length stories was extremely hit-and-miss for me. Almost the entire first half of the volume was not my cup of tea - some combination of how juvenile I found the story and how uninterested I was in the characterizations.
I did enjoy the Mistborn story - the one set at the time of the original trilogy - but not because it revealed a whole ton of what was going on in the background of the trilogy. I liked it because we got to hang out with Kelsier again, which was always a great thing, being such a lovely conflicted character. The remainder of the stories in the first half are just too silly or uninteresting for me to care about. I'm not actually a fan of the Elysium world, and wasn't familiar with the rest of the worlds there.
The second half is better, but by far the best one is the final one with Lift from the Stormlight Archive. I had forgotten about Lift (she apparently made an appearance in the 2nd Stormlight novel), and she really is utterly delightful. Stuck in childhood, but important enough to be one of the Knights Radiant, she's a refreshing breath of fresh air in a very, very serious series.
I did almost stop reading after the Mistborn tale, but I am glad I soldiered on. It's pretty darn spotty, but overall worth your time (plus you will want to know the secrets, I'm sure).