People generally agree that the published adventure is in the middle of the pack, I personally don't run published adventures as I barely find time to get through my own adventures. If you don't run published adventures either, then that's 1/3rd of this content not appealing to you.

As with any monster book, your milage will vary with the usage and quality of each creature. 60 monsters is reasonable though, although if you're hankering for monsters alone, just head over to open5e.com, a collection of products released under various licenses, collated into searchable databases like D&DBeyond does with WotC content. There may be 1000+ high quality monsters there, and plenty that fit the tone of Planescape.

So finally there is the part that I'm most interested in, the setting guide. This slipcase in it's entirety is shorter than some previous setting guides were, so if you're after this specifically, you will be recieving 1/4 of the content than you would from one of the setting guides from earlier in 5e's lifespan like Eberron. If you're keen on this product as a Planescape fan, then this will appeal to you the most and, of course, it's shallow, because it has to be on its pagecount.

I want to love slipcases. My heart is forever at the physical table using hard cover books, even when practically I DM with a computer. The slipcase allows you to only bring what you need to each session and control what content you may pass to players without depriving yourself of anything. However for me to love them, they'd need to be 175% the size of the books from 2017, but instead with the larger font size and lower page count, they're 60% of the legnth. Also of course the price is higher than ever due to inflation, which is reasonable to price them effectively, but I have no more expendable cash than I did in 2017 and post the OGL fiasco, I'm far more likely to recommend non WotC 5e content which often prides itself on being denser and deeper than official content.