Immense replayability + wide-range of distinct characters + an open world + innovative gameplay = Excellence.
JA2 has it all, and then some; The main "story" or 'situation' is this: You are asked to help an Arulcan rebel alliance to get rid of it's ruthless dictator, Queen Deidranna.
You start off by hiring mercenaries from an in-game website (M.E.R.C), and there's plenty to choose from, characters from all around the world acquiring different important abilities and personalities, and of course, salaries, so you can hire mercs only by the amount of money you have, and that's why you should get control of as many mines as possible to get steady income.
There is even a possibility to choose from between reality or sci-fi gaming style, and I suggest you play the game through both modes.
There isn't enough room for me to tell you how much good things is inside JA2, and nowadays you can get the JA2 Gold Pack including the Unfinished Business- expansion pack (which includes a level-editor) for half-price, if not even cheaper.