Hearts Of Stone the first expansion for The Witcher 3 doesn't bring anything majorly new to the core Witcher 3 gameplay but if you enjoyed The Witcher 3, and most people did, then more of it can't be a bad thing. CD Projekt Red really didn't need to reinvent the wheel here as the core game is very slick and polished. The major addition to the core gameplay are called "Runewords" , these are created by combining glyphs and provide various buffs to Geralt.
The expansion contains around 9-12 hours of new content, probably longer if you are somebody that reads every note and book and explores every nook and cranny of the DLC. The recommended level for the expansion is 30, however if you have not progressed in The Witcher 3 far enough to have Geralt at this level then fear not, you can either start a new game and play the expansion with a pre-levelled to 30 Geralt (in this option all the main quests from the core game are marked as completed) or you can use your current save and just tackle the expansion content when you are at the correct level. It really doesn't matter when or how you tackle the additional content as the expansion is not really a continuation of Geralt's story it is more of a side story.
In this addition to the main story of Witcher 3 you will find yourself seeking out a notice board, once there one of the villagers nails an interesting new quest to the board asking you to seek out Olgierd Von Eric who turns out to be one of the primary characters of this expansion. The gameplay then fleshes out to be much the same as the original throwing quests and side quests at you all of which are entertaining as long as you were still interested in the world and characters of The Witcher 3 by the time you finished the main campaign. One of the highlights of the expansion for me was a quest involving a certain over the top and boisterous ghost. The new quests are innovative and very entertaining but one particular one that involved assembling a team to pull off a heist built up excitement but then played out to be quite a disappointment.
If you liked the original game then you are going to like the expansion. Is it a necessary purchase? For me no, by the time the Witcher 3 concluded I had already had my fill of Geralt's exploits, in fact Geralt has always grated on me as a character, others who like Geralt will most likely want to snap this up especially at the very generous price point.