Short history of the series
The series had success mainly because of the gameplay. Based on older sneaking games like Thief and Metal Gear Solid, in Splinter Cell (the first one) you had to remain invisible by taking cover, using the darkness or choosing hidden paths to go forward in the game. You could attack only when it was really necessary otherwise you had to use diversions to pass the guards.
Pandora Tomorrow introduced the multiplayer mode for four users. This mode was totally different from the single player by graphics and by gameplay. Only two users were allowed in a team. So actually this was a two-on-two multiplayer mode. The teams were differently equipped. One of them could be agents and one mercenaries. The agents were equipped like Sam Fisher and the other ones like in a normal FPS with lots of weapons.
Chaos Theory evolved the multiplayer and introduced a co-operative mode. While the multiplayer remained totally different from the single player in terms of graphics and gameplay, this mode could be played similar to the single player mode. The only difference was in the different moves that could be performed only by both players working as a team.
Double Agent comes with a new concept of gameplay. This concept is based on the morality of the user. They created key moments in the game where you have to choose from several options what to do. Being a double agent in the game, Fisher works for two agencies and is forced to do tasks from both sides. While you are still working for NSA, they will give you a major mission to infiltrate and work undercover for JBA. You will encounter very delicate situations where you have to choose whether you kill the enemy or not. Depending on what you choose, you will gain or lose trust for an agency. There are two indicators for every agency. If one of these indicators will become empty it will be game over. In addition to this your actions must not reveal to JBA that you are a double agent.
PC version versus XBOX 360 version
The PC version of the game comes as a conversion from the XBOX 360 version. It looks nearly as good as the XBOX 360 version if you have a powerful PC but you will encounter some low frame rates and long loading times. Sometimes annoyingly long! But even at high settings the graphics are not so polished. From the game menu you cannot set the antialiasing level. So I tried to increase it from the video card menu. All that I got was an interlaced image. I got the same result for every antialiasing level, so in the end I quit. Besides these negative aspects, players were posting complains about compatibility with different configurations and crash bugs. It seems that the XBOX 360 version is much stable and offers a better graphic.
Good and bad
While the main character is very well defined and the graphics are good, the story is superficial. The new idea of being a double agent was not well implemented and created hilarious moments. There a few examples to point out. In jail you are still carrying one of your gadgets, and you will exit the cell through a huge hole behind a poster (copy from the Shawshank Redemption movie).
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These are the only stealth missions where you have to infiltrate in forbidden areas of the compound and steal and gather informations for the NSA. Sometimes the job is hard and frustrating but at the same time tense and rewarding in the end. You will use several times the load function before finding the right way to pass the guards or cameras. But even these missions are getting boring because every time you have to walk through the same corridors and pass the same guards in order to complete the tasks. Ubisoft introduced also some puzzles to create a more varied gameplay, but it's out of place to decrypt an email by completing a sudoku puzzle.

The Goggles
Sam always had an advantage on his side: the darkness. All previously three episodes were concentrated on this subterfuge but unlike all of this, most missions in Double Agent take place in daylight. You have to forget the warm cover assured by the shadows and the night. The strategy of hiding in the dark is replaced here by taking cover behind crates and other objects or walls.

There is no need anymore to use the vision goggles. I could easily end the whole game without using this gadget although it was created as a useful tool in previous episodes. At least Sam gets sunglasses in Africa mission. If you push the night vision button, Sam will take off his sunglasses and the image will become much more shining. Putting them back on, the image will become slightly darker, giving you the feeling that you actually wear sunglasses.

Nice but useless. I really don't understand why they almost quit using the vision goggles, when in the first episode of the series they had an argue with Tom Clancy regarding it. The problem was that Ubisoft wanted to implement two functions in the same goggles: the thermal and night vision, which technically speaking is impossible to create such thing. But Ubisoft insisted on this idea which actually gave the gameplay a new dimension.
Health
Forget about health packs. Sam doesn't need them anymore. That's because his health recovers automatically after every hit if you immediately take cover. There are only two ways in which you can die. Either fall from a high position or get caught in running fire. This is a minus for the realism in the game, but at the same time makes you concentrate more on completing the tasks than on managing your health. A good decision for the beginners, but not for the veterans of the series.
The multiplayer
The multiplayer is the only thing that makes this game unique and much better than the other episodes. While in the last two Splinter Cell games, the multiplayer offered different maps with different objectives, the experience was pretty similar. If you are a spy you have to download information from several terminals and return to the insertion point, while the mercenaries must stop them. As a spy you play from third person view like Sam, while as a mercenary you play from a first person view. The spies are much faster than the mercenary and are using the shadows to take cover. The mercenaries are equipped with big guns with unlimited ammo and different detecting gadgets. These different perspectives makes the multiplayer interesting and exciting to play. Double Agent brings in addition bonuses for winning succesive matches, the ability of forming squads and play against other squads and play with or against bots. These bots are only implemented on the mercenaries side. The only explanation is that the spies behavior is much more complicated and needs a very complex artificial intelligence to be implemented. Like in Chaos Theory, Double Agent features again a series of cooperative missions. You can play with other two players as the spies against mercenaries bots. Though similar, these missions are not like the ones in Chaos Theory. These missions don't have a story and the gameplay is much faster than the single player.
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Gameplay: 83
The moral factor gives a deeper sense to the whole gameplay but affects very little the story. If you play on the easiest level you can do almost anything without losing too much trust from both sides. My recommendation is to try the other two levels where your actions will influence much more the course of the game. The main missions are varied due to the environment. These take place in different locations from jail, to secret compound and even on a cruiser.

The missions in the JBA secret compound are pure stealth missions and are the only ones that still have that old Splinter Cell flavor. Unfortunately they are getting repetitive, because you will need to walk through the same corridors and rooms to complete the NSA tasks. Because most of the game takes place in the daylight, there is no need to use the vision goggles. It's a pity, because the game loses its stealth character. The missions are created to make more straightforward the progression into the game.
Graphics: 90
Created on the new Unreal engine, the game looks as a next gen product. Sam looks much real and has a good facial animation. Look at him when he gets caught by the JBA soldiers in the secret compound. Compared to XBOX 360 version, the game looks not so good and has a lot of bugs. Maybe a better optimization of the graphics? A big minus for the unsupported antialiasing, frame rate fluctuations and crash bugs.
Multiplayer: 85
Here can be found the big change from the previous episodes. The fast pace of the gameplay and the ability to play with bots increase the chance of replayability.
Sound: 81
The soundtrack fits perfectly to the atmosphere, but in the end it gets repetitive. The voice acting is good, but the script is much lame than in the previous episodes. At least Michael Ironside still makes a good performance as Fisher.
Hardware: 78
It is obvious being a XBOX 360 conversion, the game is a hog resources. If you don't have at least 1GB RAM, better don't try it. I forgot to mention at least 3000MHz Intel processor or similar AMD processor.