Simply disappointing.
Initially you can see that the graphics aren't very impressive and there are much better looking games on the wii. Also after a very short while you will be able to notice that the controls aren't perfect. It can be hard to do particular moves such as picking your opponent up to grapple him and give him your finisher. It can be frustrating and this problem is apparent as soon as you play the game.
The good news is that there is a story mode that is interesting. This mode is just a bunch of matches that you have to complete. This is repetitive. There is also some other good news that is that there are a lot of unlockable wrestlers and some good arenas. There is a decent amount of playtime here but only if you have the dedication to go through the frustrating career mode.
Overall I think that TNA impact, for the wii, is a disappointment and follows the tradition of multiplatform games being the worst on the wii. There is some fun to be had however and any fan of TNA wrestling will have fun playing as their favourite wrestlers. On the other hand people who don't like wrestling should avoid this because this game will not change their minds.