While I love the setting, backstory, and 'flair' that the designers have added to this adventure, things are very hit or miss.
Some encounters are incredibly fun -- such as the grey angels encounter, for example, which is probably the most fun pre-generated book encounter I've ever seen from WOTC, and which is incredibly simple in mechanics to boot. Others involve a lot of the same pattern that plagued Revenge of the Giants, where the DM gets to have a lot more fun than the players by doing chain CC effects repeatedly. For example, the very first encounter in the game has a trio of harpies that can at-will AE push 5 and immobilize the part (each turn!). they also have an exponential damage AE... Kinda a mess -- can TPK players easily on the 'warm up' encounter while playing by the rules DMs should read ahead and polish down some of these problematic encounters, or remove and replace them entirely.
From a DMability standpoint, the book does OK. Parts of the encounters make a lot of sense, but lots of little details are a pain -- such as irregular dungeon layouts that are hard to transcribe to a battlemat or whiteboard, etc. Play balance is all over -- some trivial encounters are TPK material, while some 'boss' encounters are trival. At times, it feels like the book was not well beta-tested for usability by a DM and play balance for a player group.
Pacing was a bit on the low side. By having such a huge mega-adventure, it's easy for specific adventures to not hold the player's interest, and filling in the blanks can be a serious challenge.
The high points of the book are some of the well designed traps, that get players intellectually engaged. From this angle, tomb is a success.
Overall, I think this is a good effort, but it's really held back by inconsistent encounter design and some rough along the edges testing. While I'm somewhat inclined to give 3 stars, I am giving this 4 because I feel that this module does represent a large movement forward in D&D module design from the perspective of cool settings and interesting traps that actually make the traps FUN. This was a major achievement.
But I really hope WOTC can formalize their testing process and train some of the 'hate the player' out of their junior encounter designers. I know they are capable, because the company is bursting with talent... Just a question of getting the right people in to help out. Please guys, do this on the next one!