HOTS is the second installment of Starcraft II that focuses on the Zerg story as opposed to the Terran story in SCII. Without dwelling on it for too long, the installation of the game seemed a bit strange, with it seeming to install both from the DVD and as a download simultaneously for me. I have read on other forums that for some people, it downloads the game in its entirety and completely bypassed the DVD altogether, which makes you wonder why they didn't just release it as a digit download only. Anyway, regardless of how you get it on to your hard drive, it's certainly worth the wait.
The 1 player Zerg story is every bit as good as the original. In a nutshell, the story begins at the end of SC2 - Sarah Kerrigan aka Queen of Blades has been captured and continues to enjoy wearing her orange Zerg contact lenses from time to time. The woman is also in desperate need of anti-psychotic medication to treat those anger outbursts that usually results in a burst of psychic energy followed by pain and death of those around her. Emperor Minsk is still that evil dude running the show and Jim Raynor continues to look in desperate need of a shower. Needless to say Kerrigan doesn't stay locked up for long, and the game plays out like a Rentokil nightmare as you build and increase the strength of the Zerg swarm. Kerrigan is playable as a "hero" unit that stronger during the game as you upgrade her abilities. In the 1 player missions, she can be "killed" but is quickly respawned from the hatchery so no lasting damage there. In addition, you can improve Zerg units during the course of the game, increasing speed of units, increase health etc. One thing that impressed me is that the graphics looked better, and the game feels smoother, yet the specs required to play the game are the same as SC2.
There are a variety of new units for multiplayer for the different races. I won't go into detail on those here, there are plenty of other places online that describe it better than I could. Needless to say this will undoubtedly make the game more strategic than it already is.
The only potential criticism is that those familiar with playing as Zerg from SC2 will likely find many of the early missions tedious as these act more as a tutorial on playing with that race. Actually that suited me, because I never got to grips massively with them in SC2 so I found it pretty useful. If you are a pro with Zerg, just enjoy the really good graphics! I previously wrote that an additional criticism was the fact that you needed a constant internet connection to play the game (as stated on the box as a minimum game requirement). As a gentleman helpfully highlighted in the comments, this is not true. You don't need a constant internet connection to play the one player campaign, but any achievements you get will not be saved to your Starcraft account.
Although I've read criticisms about the price considering this is an "expansion", I actually think the game is reasonably priced given the length of the one player story and the additional multiplayer units. It's clear a lot of work went into this game. I should also note that a previous review have said they had problems connecting to Blizzard's servers. I have had no such issues so I would put that down to the first day rush of people logging on. Is it acceptable? Not really. Was it likely they would have problems? After the shambolic Diablo 3 launch by Blizzard it seemed highly likely. Whatever the issues it seems to be sorted now though.
Overall, if you were fan of SC2 and found the story engaging this is a fantastic expansion that is just as gripping as SC2. There are plenty of cinematic cut scenes that look sensational, as well as the smaller cut scenes that help the story along. The improved graphics and smoothness of the gameplay are also impressive. For those of you more interested in the multiplayer side of things, the new units for the factions do spice things up. I look forward to the final expansion with the Protoss called Legacy of the Void. Game director Dustin Browder has said "we will certainly do our best to reduce the time between expansions" so I expect this final installment will be released around 2025. Until then, I would say any fan of SCII should add this expansion. Great fun.