Honestly I can't say there is much replay value(Currently no mod tools though you can play through a 2nd time and try new tactics/strategy.). Though I could recommend it if the price comes down/sale.
Played on the default difficulty(Think there was 3, I played the intermediate of the two) and the game took quite a while, though I'm not sure about to exact time. I would say it took me at least a day worth(24 hours) of play to finish it.(Unless you play it all day on a weekend, you probably will not finish it in one day).
Just a note I used more stealth in beginning to midgame and focusing more on combat later in the game(Had more ammo, more difficult/slower to evade when you could shoot a guy in the head). Choice in tactics can make playtime vary.
Difficulty varied. In the beginning to mid game, you could still find yourself running low on ammo on some weapons(I used stealth/takedown when there were too many enemies and you were outgunned to save ammo for sticky situations). Later in the game ammo is not much of an issue(Exploring for pickups in missions, picking up enemy weapons, etc). Ammo I felt is balanced, most of the weapons are balanced(Although I found Revolver with Explosive bullets to be very strong). Every weapon has its uses through out the game(I personally liked using the standard pistol with upgrades.)