Unfortunately, Kerrigan's overwhelming strength is almost a detriment to the game. Playing on normal difficulty, she rarely needed support units, except in more advanced missions. One of her abilities allows her to heal herself and allies, which let me slice through enemy bases quite easily. Experienced players will want to start on harder difficulties to ramp up the challenge.
The AI also struggled with pathfinding in narrow environments, which become common towards the endgame. I'd have Ultralisks stuck behind tiny Roaches in an attack, and they could not find a way to the target. I had to manually direct them around the combat to get to the rear or other targets. Another annoying AI trait was the need to pull units far away from combat if you wanted to withdraw. A few times, I tried to pull Kerrigan back from battle, only to have her turn around and jump back into the fray. She died a few times when I didn't notice she had reentered the fight.
That said, the campaign is solid. It took me about 12 hours to complete the game, with only the occasional reload because of a foolish mistake or a poor build strategy. The missions were challenging when I didn't overuse Kerrigan. There is a good variety as well, from timed missions to hero missions (with just Kerrigan or other Zerg leaders). There was even one mission that allowed you to control Raynor's old ship, the Hyperion, offering a nice change of pace from the constant, build-attack-defend style of play. I also found myself replaying missions occasionally just so I could get a few of the achievements I missed the first time around, giving me a chance to try new strategies as well.
StarCraft 2: Heart of the Swarm is a more-than-competent follow-up to Wings of Liberty by introducing new units and an evolution system to the solid RTS basics Blizzard has established. Just don't expect much from the story.
Kerrigan talks to Abathur when she wants units evolved.