To make a long story short, it is one of the few games that have enough strategy and tactics in them. The only other one I can think of is probably X-COM (UFO: Enemy Unknown and its sequel Terror From The Deep).
The combat system is excellent and it's realistic enough too. This is the only game I know where experience means exactly what it should - how well your soldier reacts in various surprising situations and such, not just some universal measurement of his "coolness". Such things as sniping from roofs, hiding behind obstacles, surrounding the enemy - all this is possible. If you fire your gun a lot, you gain accuracy, if you run/walk/crowl a lot, you gain physical attributes, if you carry a lot of heavy things, you gain strength. None of this "I'll kill a lot of enemies to gain experience then use it to raise my charisma and scientific skills" crap. One lucky shot in the head may kill the enemy - but may kill you as well. If you are wounded, medikits don't magically heal you - they just stop you from bleeding and loosing more health. Very realistic and at the same time enjoyable.
There is a lot of interesting characters and places to visit. There is a lot of useful items. There are many weapons too, although the game is obviously tuned to use rifles, so other weapons find little to no use at all. But you have a lot of rifles here, so it's not like there's no choice at all.
All in all, a good game with almost no disadvantages. The only thing I really didn't like is that it has some sort of time limit. If you don't take any action for some time, you get a letter from your boss and your reputation drops, which is bad. I realize that it is probably done to prevent infinite training of your soldiers, but sometimes it doesn't even give you enough time to heal everybody, especially if there are badly injured.