it looks and plays a lot like Chaos Theory, but is still a great game with a good storyline and fun gameplay.
--Gameplay--
DA plays almost identically to CT, not that that's a bad thing. You keep the standard hide-in-the-shadows that made Splinter Cell from the beginning, and all of Sam's fancy moves. You can grab and interrogate enemies. the L1 nonlethal and R1 lethal attacks that were in CT are still here. L1 will punch, choke, or shoot the "alt-fire" if you will on your gun. R1 uses the knife to stab or slash, as well as shoot your gun.
Sam keeps all the fancy kill moves that he learned in Chaos Theory, such as hanging onto an edge and pulling down an enemy above him or standing in water to grab and drown someone on land, but the level design makes these actually useful. In CT you had to almost go out of your way to use either of those moves, and even if you did you could only use them a couple times, in DA however you'll find its actually easy and fun, and a lot less rare.
The game is also less linear than previous installments, while still giving you direction. There are multiple ways to reach a target; different paths, or different strategies. In the end it doesn't feel as staged as Chaos Theory where you would go along a hallway and it would dead end...but wait! a crawlspace! In double agent it seems like you're finding the paths as opposed to them being placed for you.
--Graphics--
The Graphics in Double Agent are really good for the PS2. The environments are fairly well done, but not fantastic, and have interesting things like snow that weren't used often in CT. Also, the game is not as dark as the previous games, allowing you to see a lot of the places without looking through the grainy night vision so you can actually notice them. As usual the lighting is excellent, a staple of the SC franchise. The animations steal the show, however. Sam moves fluidly through all his acrobatic moves and close-quarters fighting. My personal favorite is his new animation for knocking out someone sleeping in a chair. Those slackers just got Knee-to-the-Face'D.
The only major flaw i saw in the graphics was the in-game faces of most the characters. It isn't much of a problem because most of the time you're looking at the back of Sam's head and he doesn't have to use facial expressions. But when there is a cutscene using the in-game graphics...well they don't look human. Their faces don't move, their lips don't sync with the words, sometimes their mouths keep moving after they stopped talking. On the contrary the CGI cutscenes look incredible.
--Story--
For the first time since i started playing Splinter Cell the story has actually intrigued me. In previous titles it just seemed like it was there so that i could enjoy the gameplay. In Double Agent you follow Sam as he goes into a bit of a crisis and goes to jail as a double agent to get his way into the JBA. Key events in the game change the story as it progresses, as well as adjust your point on the "Trust Meter," a meter that shows how much The JBA and Third Echelon trust you. While this doesn't effect too much in the game, some of the choices are really hard to make...I still can't believe I killed all 2000 people on that cruise ship...
Overall this game, while not revolutionizing the series with new gameplay the way Chaos Theory did, perfected the things that Chaos Theory started.