Graphically, it was certainly a big deal at the time. To my eyes, it still looks great today; then again, I haven't bought into any other FPS games of the last 5 years or so, so maybe my perspective is a bit old.
I like the atmosphere and there's an actual story here; not a great story, mind you, but you do get the impression that the levels weren't just thrown together in one package. Some of the AI conversations can be quite funny, too.
Like other story-based FPS games, the level maps eventually funnel you toward a certain goal, but they still leave a lot of room (at least on the outdoor levels) for taking different approaches to the objectives. There is just one rail-shooter section that I can remember.
Combat itself is not quite the typical run-and-gun style of many other shooters, and pseudo-stealth and planning do pay off. The AI guys have fantastic eyesight that may prove frustrating, but I do like the emphasis on noise and hearing.
If there is one problem with the game, it's that the story takes a break near the end and then the difficulty ramps up a LOT. Kudos to those who can get through to the end on regular difficulty, and I bow to those who do so on the higher difficulty levels. I finally had to restart on Easy. Thing is, this didn't exactly make the first 80% of the game a cakewalk, so you're not going to snooze through the game on Easy.
Anyway, I like the game. Perfect? No, but still a load of fun.
Here's the single-player demo:
http://www.fileplanet.com/135957/130000/fileinfo/Far-Cry-Single-Player-Demo