In Hearts of Stone (HoS), you'll meet men with eternal life and men without hearts.
I don't want to spoil anything, so I'll keep this review short.
I'll just say that HoS is a tale of promises, dreams, and unrequited love. The motto, if the game had one, is "be careful what you wish for."
Intriguing from the get-go, HoS' story is arguably even better than Wild Hunt's (the main game), possibly because the expansion is much shorter at 10-15 hours of gameplay and delves deep into a small cast of fascinating characters. This cast includes one of the best villains I've encountered in Witcher 3 -- possibly the best.
Among the memorable sidequests, my favorites involved a delightful village wedding, a high-society heist, and reliving the heartbreaking memories of a ghost.
There's also a new side-romance to pursue: a returning character from The Witcher 1. While each romantic prospect in Witcher 3 has her distinctions, this one is strikingly relatable and seems like a person from our own world -- maybe because she isn't a sorceress, for a change!
New monsters emerge too, and they're a bit more challenging than in Wild Hunt. For instance, arachnomorphs -- giant spiders, basically -- swarm in groups to attack you but also scatter in quick retreat, forcing you to pursue them.
Accordingly, HoS comes with new upgrades to help with combat. Swords and armor can now be enchanted with runewords and glyphwords, which grant bonuses and spells. However, only a handful felt useful to me, such as "Protection" (a Level 2 glyphword that grants a Quen shield whenever you enter combat) and "Preservation" (a Level 1 runeword that prevents grindstone and workbench bonuses from expiring).
Getting a runewright to enchant your gear can be pretty expensive. Well, you finally have something to spend your excess money on from Wild Hunt. But I recommend not splurging it all. Save some coin for the second expansion, Blood and Wine.
But again, the real meat of this expansion is its stories. I don't want to say anything else less I let the cat out of the bag.
Except this: I enjoyed Hearts of Stone more than I did Blood and Wine, for its moving tale and unforgettable villain.
Completed the expansion in 11 hours with a level 38 Geralt.