Double Agent was actually great, but the only way to play the true one (IMO) is on Xbox, PlayStation 2, GameCube, or Wii. This version was made by Ubisoft Montreal who developed the first game and Chaos Theory. It is at the same quality level as those two.
The version of Double Agent for the 360, PS3, and PC was developed by Ubisoft Shanghai who also did Pandora Tomorrow. Like you said it is okay, but it is pretty blatantly different from the other ones. Different engine, different vibes in general, and honestly just inferior in most ways.