Definitely worth the $15. If you've played this game on iOS or Android, skip this paragraph. Plague Inc is a world-ending virus simulator. It started out on iPhone a few years ago and was a huge success. It consistently topped app charts, had a number of really cool free updates, and it just gives you so much to do. The CDC even liked it enough to meet with the app creator and discuss how to make it more realistic! In this game, you control a plague (bacteria, virus, fungus, man-made plague, brain worm, and more...there's a lot to choose from). You choose where the virus begins, and you watch as it spreads across the world. You control what vectors the virus uses, how the virus defends itself against temperature and medicine, and what symptoms the virus produces (oh, and I'm just going to use "virus" as shorthand for "plague" just because I'm more used to typing "virus"). There's a lot of strategy involved, whether you try to create a nearly invisible virus that, when detected, it will be too late or if you try to create a virus that becomes scarier and scarier as it spreads and just overwhelms cure efforts. The game has been balanced to really support both options. As you infect people, new countries, and kill people, you gain DNA points that you use to alter your virus. The scarier that your virus is, the more you're rewarded with points, but the more that every scientist in the world focuses on curing you. Don't worry, however - the game does a great job of explaining everything to you. Whereas a game like Pandemic can be really intimidating with all of these different medical terms (even one for runny nose - are you kidding me?), Plague Inc prefers to use the common words for symptoms. You're not overloaded with choices (but there are enough for you) and the impact each DNA change has is clearly shown (such as infectivity, lethality, and how scary it appears to the world).
For those who have played it on mobile, not much has really changed. Each plague is unlocked after beating the previous one, there's the world map, unlockable gene bonuses, etc. However, it doesn't really look too polished as of yet. The UI feels outdated and inexplicably cheap. I almost feel like I'm playing a flash game in my browser. The UI is mostly functional, but has a lot of empty space. It also has some elements that are just for show rather than solving a purpose. For example, a bar graph to the right of the country status indicator (the main bar on the UI) has different colored bars that don't represent anything and don't move. I don't know why they're there. It's still early access, so that may change. Also, every country has a picture of it (mostly just showing people and buildings, maybe a landmark). The pictures look like cheap CGI (god knows why - there's no shortage of real pictures of locations), but there are maybe 10 - or less - pictures used for every country in the world. I don't mean that, say, Sudan cycles through 10 pictures. I mean that Sudan, and "West Africa," "East Africa," and "South Africa" (along with most of the rest of the continent) all have the same picture. Cheap CGI with nothing moving. At first I thought it was a neat touch, but that kind of shallowness stands in contrast to how fleshed out the rest of the game is.
On the upside, each plague type has its own 3D image that reacts as you upgrade it. It doesn't change as dramatically as the neurax worm on mobile, but it will stretch out and rebound a bit when you upgrade it and wince when you devolve a feature. I think that's pretty neat, and it doesn't feel as cheap. The DNA upgrade menu is definitely an improvement. Also, the world status menu has been solidified and now has all of the important info on one screen. That's a huge improvement.
Also, this game has Steam workshop integration, meaning that people are submitting their own plague types. I saw one for a Hank Hill virus earlier today. It looks like people are going to have a lot of fun creating and customizing their own plagues, and I think that's great!
In summation, it's the same as the mobile game, but with a new UI (with its ups and downs) and steam workshop support.