Righteo, I havent rated it very high, because it's not finished and I'm trying to not think of it as a polished piece.

Vision:
3 and a half because as I mentioned above, lol, but also I feel there could be some really nice line work done on this. When I saw you colouring it in yesterday, I didn't think this, but seeing it now, it's crossed my mind that you could really go to town on this lineart wise. Not too harsh, thick lines, but some with a little bit more flair would look good and seperate the colours well. =engelszorn's Inkpen Photoshop Brush is an amazing inking brush that I think would work well here. It's a little more of a natural shaped brush, which differs to the good old hard round defaults. You know my Sheva piece? That was done with this brush (and my skills were rusty there, imagine what you could do with this now!). Use it freehand, though, as the pen tool can somewhat ruin it's effects (as you can see this in places on my Sheva drawing).
The colours are great, as you know from what I've said - the fact they're vibrant all add together to not detract from the other. In a sense, the vibrancy fights the vibrancy of another colour, and so reflects the sense of action in your piece.
The perspective and composition is good, and I like the hand coming away from Sonic, it looks good.

With Eggman's hand, I think the lines feel too solid and spindly to reflect the action of him throwing (is this correct?) and it needs a greater sense of motion. Try looking at photographs of people blurring when moving, or do it yourself in a badly lit room, with your own camera. It may be harder to pull off, but no-one can blame you for trying .

I like Tails', er, tail, that has a nice feel of action to it aswell, but maybe blur the white around the tail (like the smudge, but with a brush...this can be done when you shade anyway) to give it more of a spinning feel.

Originality:
Four stars because it's not a copied pose. Only missing the one star because it's fanart, of course! But that's not to be frowned on, it's just limiting in my rating because it's not your own characters, which I'm sure you'll agree with.
The poses, the scene, the actions are all original to me, plus you've used your own style and their colours to create a highly original piece of fanart.

Technique:
It's not done, but so far it's good. There's a few patches where the flat colouring is leaking through the lines, but as your description says you're not finished with the colours yet.
Of course, when submitting the real one, your technique will go up due to the shading and lighting being finished

Impact:
It shouts out to me, it has a sense and thrill of movement and action and the colours bounce off one another in a good way.
4 and a half

Not much more I can say, mostly it's down to your lines and that's it really.

Have fun - shading, in my opinion, is the best bit!!

Happy trails! (Whatever that means)