SCDA is definitely the best in the series, highly recommended for SC fans and people who loves stealth gameplay.
The best thing about this game is the graphic. It might not be very impressive at the first 3 levels of the game, but as you get to level 4 and so on, the game starts to blow your mind with never-seen-before graphic effects. A great thing about the SC series is showing off the graphic power of a console ( alright, the xbox ), and this time SCDA proved that XBOX 360 is the one with the most power, not even any current PS3 screenshots can look this good. This is next gen, no need to wait for PS3.
As for the gameplay, no big innovative here, which means it's basically the same as previous SC games. It can be good and bad. People who are expecting new style of gameplays will be disappointed. However there are new changes, Sam Fisher is now spending a decent amount of time in the daylight, and upgradable weapons and the double agent elements do make the game feel new and more interesting. The bottom line is if you are a SC fan, you will definitely love it.