I think WOL is better, yet this is a very good game. To use an analogy WOL feels like a 120 minute movie with good pacing, while Heart of the Swarm feels like an 80 minute movie with good pacing.

One thing I felt lacking is the Kerrigan allies/npcs are much more one dimensional than the crew of the Hyperion. Tycho and Matt feel like partners while Kerrigan's allies are merely minions and thus don't have character.

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I highly recommend the game.

One thing that is different is each level has 3 achievements that you have to complete on normal difficulty (I have practically all of these done right now). Each level then has a mastery achievement which you must perform on hard difficulty.

Do not worry about any branch missions for Heart of the Swarm lacks these. Also go ahead and finish the game, just make sure you do all 27 missions. Once you finish the game you unlock the "master archives" where you get to replay the levels again. Furthermore when you replay the level you can reselect all your kerrigan and army abilities including the "strain" you pick for zergs (you liked jumping zerg the first time, why don't you take the evolution that gives you 3 zerglings this time.) Finally Kerrigan will have her maximum level for the master archives as if you completed all the bonus objectives in the prior missions. You can finish your achievements while doing the master archives.