I tried "Visiting Exhibit" when I first got Dark Pharaoh, and had a lousy game. From thenafter, I always shuffled everything together. When Dunwich came out, I didn't even try to "see all of Dunwich first" and just mixed them all in too. Discovered quite a few things after that:
1) I didn't like Dark Pharaoh at all. But that's another thread entirely.
2) I prefer not to spoil a new expansion too quickly. I was able to drag out the "reveals" of Dunwich for weeks, as opposed to burning through them in a few games, as I did with Dark Pharaoh (accelerated by "Visiting Exhibit"). I will never scan a "new" deck right out of any box (well, not more than 3-4 cards anyway).
3) Mixing expansions together tends to breed trouble. The "Dilution Factor" seems to be more of a personal choice of playing style, but the fact that every expansion only plans with the Base Set often produces conflicts that have no answers (the old Rift-in-a-Vortex gambit). Some like this trouble...I do not.
When King in Yellow suggested I stack its cards on the TOP of almost every deck...seriously?...I thought that sounded ludicrous. What about Elder Signs? Or simple Withers and Mists of Releh? Or just the basic Mythos cards, the ones that clear a neighborhood, or dump two monsters in the streets? I wasn't sure I wanted THAT MUCH "concentrated" Hastur. But I violated Rule #2 up there, determined to try it. Sure enough, my first game gave me Investigators with a lot of starting Common Items, and everyone had a Gladius. I played four turns and was severely annoyed with the complete lack of (I guess) "normal" Arkham cards to blend with all this Carcosanity. I quit, started over (shuffling everything together), and had a fantastic first game.
Kingsport and Black Goat have been shuffled into the rest since their first games. (Not always together: I stand by the "One Board Expansion and one Card Expansion at MOST" playing style.) There are still a few Kingsport cards I haven't seen, because "another one that I haven't seen" keeps popping up from time to time, and I bet I've barely seen half of Black Goat by now. Plenty of surprises still to come.