First of all, most of the other reviews here are just confirming that they received the key, and not information about the game, which is what is supposed to be here to guide potential buyers.
It is a great game with a fantastic competitive mode with a huge global player base. I would recommend playing with at least one other friend in competitive as it might be hard to coordinate tactics with strangers on the internet. One area that CS GO is lacking is graphics. This is actually a good thing for the e-sports games as it means you can play it on pretty much any computer build in the last 10 years and can easily push hundreds of frames per second for a smooth and responsive experience (providing that you have a monitor that can actually display them all). If you have every played a Source engine game, you will pretty much know what the movement and gameplay feels like, and if you haven't, I would highly recommend the Portal series (not that it is related to CS). One final thing to watch out for is the online gambling scene for gun and knife skins for CS GO. These can be very expensive and I would not recommend spending money on any 3rd party skin gambling website as they have extremely low chances of winning. Through Steam's community market, you can get a large variety of finishes for your guns and knifes to show off to your friends, but try not to spend too much on them and remember they are purely cosmetic changes to the weapons.