Note: As usual, I haven’t played multiplayer yet, and I may not. This strictly covers the single player version of the game.
Heart of the Swarm, of course, is the sequel to Starcraft II: Wings of Liberty. It contains 20 Zerg missions, and follows onto the story from Wings of Liberty. There’s a tiny bit of Mass Effect action going on, in that choices you made in WoL can affect you in HoS. Nothing major, but it’s kind of fun all the same.
HoS is more an RTS+RPG; it reminds me of Warcraft III in that way. For most of the game, your hero unit is Kerrigan herself. She’ll get more powerful as you complete missions, both the main and optional objectives. These abilities range from a nuke-type attack to zerglings that revive to getting two drones at a time. In addition, every unit in the game has several mutations (which you can change between missions) and evolutions (which you access by completing side missions).
One area WoL did better was the characters; there’s nobody terribly engaging for my money, and it lacks a little of the charm from the first game. Now, that’s not necessarily a bad thing, and is probably intentional, given the more serious nature of this game. The overall story is solid, with a very reasonable progression from the first game and a clear direction from the third game. The twists are kind of predictable, but still done well.
The levels all feel different, and play differently. It isn’t as innovative as the base game, but this is an expansion pack, and what it adds is plenty worthy of $40 (or less, as I had some Best Buy gift cards). You genuinely feel like you’re in charge of an absolutely massive horde. In a couple of levels, there were so many units my game actually slowed down a touch. The game itself recommended I turn down my settings, which was unexpected. I didn’t (everything to the max!), and the mass unit problem solved itself in the usual manner: lots of carnage.
The game has only one weird bug I could see: between the first and second mission, you get a black screen. If you ALT+F4 out and reload, you can continue fine, but it’s just weird that that happened in the first place.
Definitely worth picking up if you liked Starcraft II.