For the most part, my first impression is that I love this module. More than H2, in fact. Why? Because it's such a blast from the past, but at the same time also perfectly modern.
Allow me to expand on that. Have you ever played 1st ed modules like White Plume Mountain, Ghost Tower of Inverness, or the like? Are you nostalgic for that sort of thing? Well, your dream just came true. The Pyramid of Shadows is a series of rooms that each have a certain theme, trap or trick to them, designed to test the player's puzzle-solving skills and not just the way they flick their dice-hand wrists. Every single encounter has some funky feature that sets it apart - there is not a bare dungeon room with 5 orcs in it as far as the eye can see. (OK, the bandit's lair comes close, but even that has the you-know-what locked in the you-know-where) Heck, getting to the final level involves solving a bunch of silly riddles! Man, that takes me back...
What's modern about it there's an explanation for why all these wildly different beings and environments rub shoulders with each other. Granted, it's an explanation that only makes sense in a D&D universe, but that should be good enough. More to the point, every area is an object lesson in 4th ed encounter design. All the creatures interact with the environment and use it to their advantage. And there's none of this H1 "room with 3 traps that can all be destroyed safely from range, with no monsters to back them up" crap either. In fact, there's quite a few nasty surprises in store for the players, mwahahaha!
(Digression: if you hadn't guessed, I am the kind of DM who feels that a good, fun encounter is one where I manage to knock at least one PC into the Dying condition.)
So, overall a great job from the designers. I have a few niggling points of criticism, though.
There's an encounter that feels recycled from the DMG adventure. I guess I can justify it as "revisiting the same basic idea to show the players what difference a few levels make."
The art you're supposed to show the players is in many cases very dark and muddy. It's hard to make out what it depicts unless the light in the room is very bright and you show it at exactly the right angle. Also, the "folding the page in half" thing would have been easier if the pics were actually in a separate little booklet of their own (like in the old classics).
The number of magic items handed out is way below par. I only count 9, as opposed to the 16 there should be. Now, it's possible this is balanced by a higher gold allotment, I haven't been bothered to count all the cash, but if the designers pull a switch like that I expect to be told about it up front. Anyhoo, it's not a huge deal, as the exact items given out always have to be modified in any published adventure anyway, and I can easily add a few more while I'm at it.