Yuck, what a mess.
Graphics
Double Agent on the Wii is ugly, very ugly. The game looks like the PS2 version, only at a very low resulution. The shadows are a mix of black white and strangley purple. The characters themselves look horrid. The framerate is very slow as well, and when there are more than a few characters it almost comes close to halt. What I don't like is seeing the Wii get a watered down port like this. The Wii may not be able to render things in HD but the Wii is still pretty powerful when it comes to graphics, and some polish to graphics engine(a major polish) would have been appreciated.
Sound
The game strangley sounds weird. Like Ubisoft went back and toned the sounds of game down to mono. Alot of the voices have a strange muted like quality to them. Also strangley, some of the sounds aren't even in sync with the game at all, nor is it even there at times.
Gameplay
The Wii mote control does actually save this game from and utter disaster. The wii mote surprisingly responds very well to the game. Controls are mapped out for the most part pretty well. But what is the major flaw in the controls is that Ubisoft failed to speed up the aiming axis when you withdraw and use weapons. In all actuality the aiming is terrible. On many occasions I botched the mission because of the terrible aim.
Overall Splinter Cell Double Agent is a great game, just not so much on the Wii. If you have other consoles, consider playing it those. This game pretty much came out for every console, including the original xbox, so your better off playing it on other systems.