Here is Metal Force Knuckles.
As you can see, he still has his light-up emerald rod, and strange armor. ALL the figures are in identical armor. The armor has been painted differently, in all cases. The texture of the armor is also identical. No Metal Force figure has a helmet. Other details are missing, for example Knuckles does not have the orange fin at his hip, and Shadow does not have a fin on his leg back like the Space Fighters one. See the original Knuckles review here to compare.
Here are 2 comparisons, so you can see how each figure's color scheme differs, and how Knuckles looks without his strange helmet. The Metal Force 'space rod' is also a slightly lighter color of plastic, but this does not effect it in any way.
At the close-up of his fist. You can see that ALL colors on all metal force figures are 'pearlized' versions, even if it's hard to notice in the photos. Even the red on him has a pearl-tone coat. The golds and silvers are all appropriatly metallic. They DID make an honest effort to reign in overspray, and for the most part it worked.
Over all, the metallic paint is a nice touch for these figures. It changed their color drastically in some places, and really improved some of the suits. Removing the helmet for Knuckles and Tails was a nice move, as their heads had a good sculpt. Shadow and Sonic...it just reveals that their heads are a poor job.
The Knuckles on this line CAN have this problem, likely the bane of MIB collectors. His head isn't snapped down. See how the eye/nose piece is separate from the head top/spikes piece. Here, the head top isn't snapped down all the way, exposing material above his eyewhites and making the figure look strange. This is fix-able though, if you open him, you can push the head top down to snap it into place & the figure is fine.
At right, you can see the Tails with a similar problem. In his case, it's the bottom white part of his face separated away from the top/yellowish. Just like Knuckles, he can be opened & snapped together properly.
As you can see, Metal Force is a repaint of the Space Fighters line, and doesn't include Missile Wrist on the line. Also, the figures do not come with any headgear. They have also corrected their box-back photo to show Shadow's sword as it really is: one piece with no handle.
Metal force is available from Toys R Us, in most areas, as of November 2006. In 2007, they can be found at some KB Toy stores in the USA, and also at some online retailers like Amazon, possibly.