I'm a noob to the forums here, so my reaction isn't nearly as deep or exhaustively analytical as many of the others here. Sorry bout that.
My main reaction was that I had an absolute blast reading Secret History. I knocked it out in one session (not that impressive) and just had so much fun with it. I'll probably have to go back a re-read it, but for the first time through I just sat back and enjoyed it. I don't mind at all that Kelsier was "brought back from the dead," because he's a cool character who's extremely resourceful and in a cosmere jam-packed with supernatural elements, I practically expected him to.
My other thought was that I'm SO HAPPY it finally explained Vin's reluctance to visit the informant. That has driven me nuts for years, and I was just reminded about it when I re-read the original trilogy a couple of months ago (now I'll have to re-read White Sands because I can't remember Khriss at all haha).
I loved learning so much about how everything works. I know there are a lot of theories and such out there, and various little tidbits dropped here and there over the years, but it was amazing to have so much of it in one place. Really opened my eyes to the depth and complexity of this thing B-Sand created.
I loved it. Good stuff. Fun to read. I'm interested to find out how old Kell ends up back in the Physical Realm. Other questions (which are pretty standard): Why the heck are the plants in the cognitive version of water? What was the endgame of the Elantrians? Is Hoid a good guy or a bad guy? WHO IS SPANKY???? (jk) Can't wait to find out!