The Saboteur is a fantastic game! You play as an Irish terrorist named Sean Devlin in Nazi invaded Paris in the year 1941. This game is a huge melting pot of other games thrown together wonderfully. It has elements from Metal Gear Solid, Assassin's Creed, Grand Theft Auto, and Hitman.
The elements from Metal Gear Solid are that you can be very stealthy. Take out a guard, take his clothes, then walk around as a Nazi without anyone else the wiser. Very realistic though in the fact that if you are in disguise, then run full speed or climb on things, the enemy will out you and been firing. Or if you are near them face to face to long they will also recognize you're not one of them.
The things that this game has in common with Assassin's Creed is that it it is freeroam and you can climb. While not AS fluid as Altair or Ezio from the Assassin's Creed games, Sean Devlin can climb fairly fast and maneuver well. You also do have some brutal fighting as well as a couple stealthy kills. Pressing Square will have you snap their neck, whereas pressing Triangle will bash them in the head.
As far as the GTA similarities... YOU CAN STEAL OLD CARS! Seeing as this is a freeroam sandbox style game, you can run around Paris and steal cars to get where you're going. Thing I like that they added to this game though, is that when you steal a car and deliver it to your garage, it will permanantly be there. Then talk to your garage attendant and you can pick between which of your cars you want to use. Another similarity is that in GTA you had to find hidden packages or other items scattered throughout the city. In The Saboteur you actually have spots you have to blow up and Sabotage all throughout Paris. Whether it is an AA Gun, a Searchlight, Radar Beacon, Sniper Tower, whatever really, there are MANY of them to find as well.
The Hitman similarties are fairly obvious. You need to be quiet, clean, and smart to get through some of the harder missions. You can't kill someone and leave them there, it will alert the other guards. You need to be a quiet assassin or you will be compromised. Shooting a gun or even aiming it at someone can alert the guards. So if you want to shoot someone, get a silenced pistol and take the guard out fast.
The Saboteur has some of it's own goodies to this play style as well. You unlock perks as you kill and maim the Nazi's! If you run over 30 nazi's in a car, you unlock the ability to put a nitrous boost to a specific car. Or if you fight with your bare hands a lot, you unlock a haymaker as well as a special throwing ability. In the "Demolitions" area of the perks, you can make your explosives blow up faster, or it won't be suspicious to the guards.
Overall, I would highly suggest this game. It is a great overall game with a few glitches here and there. And for the price tag of $19.99, there is no excuse not to try it.