I find myself starting to lose more and more interest in this project each day. Mainly because the mistakes here and there are too much. I'll start from the top.
Sonic seems to be drawn in the Heroes and/or Next-Gen style, but yet his spines are shorter than the ones on SSNTails's sprites.
Tails seriously looks bad. He's too tall, and he isn't as cheerful as he usually is.
Knux actually does look alright, except for one thing I'll say in a sec.
Shadow is the same as above.
Amy lacks serious polish. Her head is way off, and so is her dress. Both of these could do with some serious fixing.
Silver's shading needs work. Especially on his chest and back.
Rouge doesn't look nearly as "sexy" as she usually is. Her eyes are closed too much, her hair is too long, her wings look like they're running off from the hair, and she lacks "t3h bewbs and ass" that give her that Rouge-ness.
Mighty looks horrible. His design is way off. I don't need to say anymore, so fix it or forget it.
And finally. The majority of the characters look old and tired, which is hardly the high-energy that the Sonic series has