Kobolds and Catacombs is a new expansion for Hearthstone, or as Blizzard likes to call it, a new cash cow expansion. We have a new set of 135 cards themed with Mines, Kobolds and treasures, as well as fun new singleplayer mode called Dungeon Runs. In Dungeon Runs you pick your class and try to beat 8 bosses, and with each new stage you get to pick new powerful effects and cards. There is a lot to explore there and dungeon runs will keep you busy for a while.
New cards are interesting and you can find some interesting new mechanics there, and you will be able to create some very cool decks. Card upgrade mechanic will upgrade your cards for each class when you complete a set of needed requisites. For example new Rogue card Onyx Spellstone with effects destroy 1 random enemy minion will upgrade to destroy three minions if you play 3 deathrattle cards. So there is an incentive to keep those cards for late game and use stronger effect on your opponent. There is also a set of new legendary weapons for each class and there is yet room to explore all the possibilities there and all the possible interactions with old and new cards.
It is good that Blizzard keeps producing these expansions to keep things interesting, but as always, if you want to own every card you will have to pay. I like to collect dust and craft interesting cards, so pay to win aspect never bothered me in this game. It is up to you if you want to buy those booster packs. Kobolds & Catacombs is a decent expansion and will keep you busy until the next one in 2018.