.:Fionas Yellow Costume:.

Crit. Right. I've had my ass chewed out one too many times by deviants who I have critiqued that I've shied away from them. But you don't seem like the kind of person who will kill me if I say something seems "off".
Having said that, I'm no expert when it comes to art, so if I say something, feel free to take it with a grain of salt.

First off, I commend anyone who even attempts Sonic proportions--personally, I try to draw them more human-like in proportion to avoid what I call "big head syndrome". While not bad, Fiona's head does seem a little too large for her body. The details on her head are very nice though.
I'm not sure if it's because she's turned or not, but she appears to be missing a neck. Her breast also seems a little too high on her body, but that again may be because she is turned.
Her nose is a bit small, and awkwardly placed. I feel as if it should be higher up on her muzzle.
I love her pose, but I'm not too fond of the muscle line on her arm--it feels out of place almost. I don't really think it serves to enhance the appearance of her muscle.
I'm a huge fan of large, bushy fox tails, so that was a little dissappointing. Her tail is executed well, but I was hoping for something a little bushier. It comes across as cat-like in this particular piece.
In terms of coloring, you nailed the colors! I wish there was a bit more in terms of shading--as in, I'd like to see something more dramatic. Lighter lights, darker darks...that kind of thing.
Her hair is lovely. I love the movement in it. The background choice is also very nice, though the orange in the jumpsuit seems overpowered by the orange background. Also, Fiona's brown, while it looks great against the orange, might be better served with a cool color in the background--perhaps a blue.
All in all, this is not a bad piece! Sonic style is probably the most difficult style to draw because of the funky anatomy, and keeping a few minor ideas in mind will propel you into the next piece.
Overall: 13.5/20