Before I begin: Due to a cock up by Amazon, it looks like my copy of this book may have been free. I'm going to try not to let that influence my review, except for one comment in my conclusion below. Still, I felt I should provide the disclaimed right up front.
Anyway, onward...
If this book had come out 3-6 months after the 5e core rulebooks had been released, and had it been priced at $35, I would have given this book a solid 5 star review. As it is, however, this book is too little, too late, and way too expensive.
It's not that I consider the $50 price point excessive... provided they give good value for money for that. And the 256-page storyline books have mostly done that. Even "Volo's Guide", at 224 pages, was just about okay provided you got a decent discount from Amazon or similar (which sucks for the independent retailer, of course, but that's another rant).
But "Xanathar's Guide" costs the same $50, and further cuts the page count, down to 192 pages. To justify that, the contents have to be absolutely stellar - of a quality that hasn't been seen in RPG books since... I don't know - maybe since the very first edition of "Vampire: the Masquerade" changed the entire landscape?
The contents of "Xanathar's Guide" are... okay, I guess.
The book is broken down into three chapters and two appendices. The first chapter consists of character options, and is mostly made up of new subclasses for each of the twelve classes in the PHB. No new classes are presented here. The new subclasses are fine, with no stinkers that I could see. I was particularly impressed with the decision to make the Hexblade a Warlock subclass - I thought that was an inspired choice. Conversely, my reaction to the much-hyped Forge Cleric was "meh". I had read comments that suggested that this might patch the hole left by the Artificer, but frankly I didn't think it was anything more than 'okay'.
This chapter also has some additional feats, each tied to one of the races in the PHB. Those are okay. And there were some other bits an pieces about fleshing out a character's background, personality, or other details, which were fine.
The second chapter is filled with DM tools, which again range from the space-filling (we really need half a column on Cook's tools?) to the meh (knot tying!), to the really quite good actually (lots of random encounter tables, a new system on building encounters, and quite a lot of new ways to use downtime - including rules for both buying and crafting magic items). I also enjoyed the discussion on traps, except that I think they probably missed a trick here - the book notes that a simple trap mirrors a spell while complex trap mirrors a monster, while failing to note that that's probably exactly how these things should be presented (and we probably need a "Big Book of Traps" for exactly that reason).
The third, and shortest, chapter has some new spells. These are fine - I didn't feel any need for more spells, but there's no harm in them.
The first appendix is probably the single most useful part of the book, at least for my own, personal needs - it goes into quite a lot of detail about shared campaigns. But, more importantly, a whole of of the material here is easily adaptable for the sort of game where you can only play very rarely and want to skip away from a lot of the 'book-keeping' aspects of monitoring XP and assigning treasure. Good stuff.
And then the second appendix gives 18 pages of sample names.
Frankly, I'm baffled by this. Providing sample names is fine, I guess. But it's hardly the highest priority. And when the book has had its page count cut, and with the price point that marks it as a premium product, is this really the best material that WotC can find to fill it? After three years working on this fifth edition of the game, have they really produced so little useable material that they'll fill 10% of their new rules book with lists of names?
I find that a shocking decision.
So where does that leave "Xanathar's Guide"?
Ultimately, I cannot recommend this book. Yes, if you're desperate for new material, and especially for new subclasses, this book may fill that spot - and doubly so if only 'official' works will do. But other than that, and unless you can get a massive discount on the book, it's just not worth it - you'll get much better return on investment elsewhere.
Now... I said a while back that I was going to afford WotC and 5e "three last chances". Of these, the first was "Volo's Guide to Monsters", which was okay but no more than that. This would ordinarily have been the second, and likewise would have been a failure. However, as I noted at the outset, it appears that I've been given a free hit with this book. That being the case, I'm minded to allow WotC a do-over on this one - I'll give the rules supplement from next year a chance to redeem the failure of this one. (And the third last chance remains - if they do a suitable setting guide/adventure, I'll give that a go. I've also decided to expand the list of settings I'll try: in addition to Eberron and Dark Sun, add Spelljammer and "a new setting". That said, I'm very much doubtful that WotC will be formally supporting any more settings, so that may be a last chance they never test...)