Both expansions add tons of new content to an already amazing and perfect game that is so good that I would probably rate it the best game I've played - and for 154 hours so far!
It's almost hard to separate the main game and the expansions, having played all three I see them all os one big chunk of enjoyable content. The main game was superb, fantastic characters, great main story and locations. The story and the game length itself can be difficult to comprehend in all its splendor while playing, but if you backtrack and think about it having finished the game it is one hell of a ride full of epic and memorable moments.
The first DLC, Hearts of Stone, takes place on the existing world map and has a very intriguing new storyline and introduces some really good new characters. Geralt gets afflicted with a curse and offered a cure in exchange for killing an immortal bandit leader Olgierd von Everec. While the fact that the expansion only takes place in mostly explored locations is a bit of a letdown, the main quest and characters in Hearts of Stone more than make up for it with great dialogue and boss battles. Some people think that the main quest in Hearts of Stone is better than the main quest of Witcher 3, I would not go so far to agree but I really liked the motivations of both friendly characters and the villain in this story.
The second DLC, Blood & Wine, opens up a completely new region - Toussaint. This is pretty much a fairy tale oriented fantasy location compared to the wartorn, gritty and brutal map of Velen/Novigrad. The way the knightly NPC's interact with Geralt opens up for a lot of quirky dialogue and encounters. Toussaint is a pretty big region packed with small villages, quests and a large main city hub. There is plenty to do and see here. The main quest is a more "basic" monster hunt and not as intriguing as the main quest in Hearts of Stone. It does however offer a lot of really good moments and decisions for Geralt to make - especially towards the end. The ending of this expansion is both goofy fun in parts, and also offers an epic battle where you really feel that "OK we are in serious trouble now!".
It's hard to describe the goodness of both expansions without writing spoilers as both expansions are best entered with as little knowledge of the content as possible to fully enjoy each of the new storylines. Fortunately you can install and start playing the expansions at pretty much any point of your main quest or even after you have finished the main game. Each of the boxed editions of the expansions come with two decks of Gwent cards (Hearts of Stone comes with Monsters and Scoia'tael, while Blood and Wine comes with Nilfguard and Northern Realms). So if you enjoy playing Gwent in Witcher 3 you can now play it "live" with a friend. I only wish they had used the "second/alternate" version of the artwork for the cards which imo looks cooler.
I REALLY recommend getting both expansions if you already own the main game, beside the new quests and characters they also offer increased level cap, new tweaks to alchemy and mutagens, new weapons and armor that will make replaying the game with all that new content worthwhile!