Folks says its XCOM-like game, but in fact, it isn't.
Sure, you are head of secret, govermental agency, but that where similarities ends. Sigma Theory is about politics, spies and conspiracy, so you more like leading CIA operation rather than full scale war.
Your main work is to oversee researching of advanced technologies, that will give your nation edge over the rest of world. To do this, you will recrut scientists (in various ways), do some diplomatic matters, interrogate captured spies and send agents to achieve any of this.
There's a lot of great mechanincs, like every agent has characteristics, that makes them more or less suitable for certain action or scientists and diplomats may be more likely to work in your favor if you gather some intels about them first.
Sometimes you'll need to move someone from hostile country and exfiltration happens, what is cool and gives kinda 007's vibes.
But as a game mostly about politics, every failure here is very pricey. Some of them are simply unavoidable and push you into quagmire, so you have to have plan B (or even C). But actually it adds some tension and weight to every decision you make.
The bad thing is game itself barely explain you how different perks or mechanics works, so you'll probably fail twice or thrice before get the idea, what to do. Also, the pace is very high, so doing fancy thing isn't an option and casual mode isnt available.
Technically its mostly ok, with some minor annoying features like flood of popups, occasinaly glitching interface and blocks of generic text, one for every event of certain category (unfortunately it isnt Jagged Alliance tier of personalizing content, dispite attempts).
Overall its a great game in rather rarely touched genre, when you may feel like guy from 3-Letter-Agency, who command some secret operation on foreign grounds, trying to both please superioirs and avoid political disaster.
It has some problems but I have fun plaing it, so 4 stars.