Absolutely brilliant, highly addictive and long overdue for a remake... And pretty unique too.
The path to world domination consists of completing 1mission per region (usually the size of a country), ranging from assassination, persuasion (brainwashing), sweeping a city clean of enemy agents, stealing some super weapon, destroying vehicles to escorting people. More than enough variation to keep the missions interesting in any case!
Syndicate is an isometric third person shooter (you control up 4 agents at a time, either separately or as a group) with some strategic elements such as research for better equipment (weapons, enhanced armor and other body parts improving your agents' stats), choosing which country to claim first, taxing the population...
The choice of weapons is extensive enough and many of them have their own specific use: long range, flame thrower which has a neat effect on enemy agents too, devastating gauss gun, allround minigun,...
The difficulty increases gradually though the very last mission is a tough one to crack. And throughout the game you always have the option to choose from a few missions so you can start with the easiest until you've got access to better agent enhancements or weapons. In addition to the missions on foot, every now and then you'll use vehicles such as cars, police cars, slow armored vehicles and metros.
In terms of graphics, they were just awesome when Syndicate was released and thanks to the small scale and relative detail, they have aged quite nicely.
Too bad that it's not so stable on modern pc's (running it under Dosbox for example), otherwise I'd definitely find myself playing it over and over again.