This game has some of the highest replay value for me on my PC. I've played through it multiple times, and have not yet played it without finding new secrets, or uncovering new ways to approach a mission. It's a fantastic combination of a FPS and stealth gaming. There's lots of engrossing levels with multiple ways to complete. You can go in guns blazing, and many a guns you have to choose from. Shotguns, automatic rifles, sniper rifles, RPG's, flamethrowers and more.
Or take the stealthy, and often non-lethal approach, with weapons like the dart gun, riot baton, and electric prod.
In addition to your arsenal, you have bio-mechanical upgrades you can plug in, including cloaking technology, upgraded speed, strength, vision, and more.
You've got your weapons, you've got your mods, AND you've got upgradable skills! Lock picking, computer hacking, marksmanship, melee fighting, AND MORE!
As you can see, "AND MORE!" is a common theme with this game. It just doesn't disappoint.
Since you have a selection of both weapons and mods that's larger then you can take at any one time, and an unfathomable amount of combinations as to which skills you want to focus on, this also adds to the re-playability.
On top of all the gadgets, and choice in mission strategy, the story line is a really good, with lots of factions and characters to keep track of. The benefits of all these groups is apparent in the end, when you get to decide which group is going to win, opening up a multiple ending scenario, ALSO adding re-playability. I even found the voice acting (for the most part) really well done.
Do yourself a favour, and play this game. It's few years old now, but still well worth the experience.