Step down for the series but still enjoyable.
First off the game is too short. 9 levels, 4 of them in the same exact location. These 4 levels are "undercover" missions that are honestly very well done and exciting, but that doesn't make up for the fact the game basically has 5 levels. In these undercover levels you will move around the terrorist HQ avoiding being seen in the wrong places and trying to plant bugs, recover intel, and keep up appearences. The story and happenings in these levels is great, but still... they all look and play the same, and it just doesn't equal the same amount of content 4 different missions would have provided. It would have been better if the kept the undercover mechanic, but had it take place in different places everytime... the terrorists do end up traveling the world anyway.
The other 5 levels are very good though and classic Splinter Cell. The Shanghai and Africa missions are especially great. Anyone who loved Chaos Theory should enjoy them quite a bit.
Other aspects of the game are largely the same as previous games. The interface is a little different, the light meter changed to a three-tone light, which works just fine. It seems like they wanted the interface to be as small and unobtrusive as possible, which I like, but infortunately they kind of ruined it with huge and ugly action icons for when you perform tasks like opening a door and grabbing someone. Chaos Theory's were much less in the way of the scene.
Graphics are pretty good, though not a huge jump from Chaos Theory either. I ran the game in 1680x1050 resolution with all effects cranked, moderate AA and AF. Some levels look a lot better than others and in general textures on characters were better than in the game world. Still, a great looking game, if not a huge "next-gen" step.
Stability was fine for me. I am using an 8800GTS 640 in Windows Vista. I got no crashing problems or real bugs as a lot of people have reported. I can't comment on those issues since I did not experience them. Sometimes people don't update their drivers correctly, or use a pirated copy or disc crack which causes issues. Not sure how many fall under this category... point being, the game ran perfectly for me.
All in all Double Agent is a worth purchase for any Splinter Cell fan. It's not as amazing as Chaos Theory (which was done by a different team who are working on the next Splinter Cell), but it's still a great stealth game that I would put above the first two in the series as far as replaying them all right now. If you have the specs, I highly suggest you check it out.