I've run through most of it (the PCs were in the early parts of the Weeping Colossus level, getting close to facing the final Prophet and Imix, when we put the campaign on hold because we switched to online gaming on account of Covid), and I'm mostly not a fan. There are some cool elements (heh) there, but I think the dungeons are too big, and too connected into a single 9-level megadungeon with four different entrances. Since there aren't really any natural divisions other than stairs, it all feels very much like a long slog.

I'd rather have seen a situation where the different elemental temples were (a) smaller and (b) in different locations, which would create natural breaks in the action (which would then give the DM the opportunity to use the side treks more than I felt would be natural). That would also provide an opportunity to make the path to the temples adventures in themselves. Perhaps you need to ally with a goliath tribe to learn the location of the air temple located on a mountain peak, or do some underwater exploring to find the coastal entrance to the water temple.

Basically, think of how Legend of Zelda: Ocarina of Time works. The dungeons in the game are certainly the stars of the show, but there's usually a bunch of stuff you need to do before you can get to them. For example, before you can get to Inside Jabu-Jabu's Belly (the game's third dungeon), you must find Zora's domain, interact with the Zoras, learn that their princess is missing, get the ability to dive deeper via a mini-game, use that to travel through a portal to Lake Hylia, find a message in a bottle there from said princess saying she was stuck in the belly of the giant fish-god Jabu-Jabu, and then show that message to King Zora so he will give you access to the fish-god. Then you can get to Inside Jabu-Jabu's Belly. That's a lot more fun than teleportation straight from Dodongo's Cavern to Inside Jabu-Jabu's Belly would have been, which is basically what you have with Princes of the Apocalypse.

Now, Ocarina of Time is certainly more puzzle-oriented than typical D&D campaigns so I'm not suggesting one should use a sequence like this literally. I'm just saying that a bit of breathing in between dungeons is good for the soul. Oh, and the dungeons should be smaller – just as an example, the Temple of the Crushing Wave has about 18 encounters in it, which is at least double and maybe more like four times as many as a dungeon ought to have.

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