Actually, and surprisingly, not all that bad
Well actually in the Genesis days he was fighting Dr. Robotnik... Sorry, but I don't think I'll ever get to calling him Eggman, ever. Lol, but moving on
For the most part, the sprite work was very well done (though I think it got sloppier as the flash went on; this is most noticeable in the part where Sonic stormed the rocket facility). It displays a certain level of competence and confidence working with sprites, and I was very much pleasantly surprised that this did not suck.
Another point of praise was the Robotnik battle at the end. I don't know if you ripped off some design from another Sonic game, but in any event, I thought it very creative and actually pretty fun to watch. So, kudos on that.
Some criticisms, though:
-You could've spent some more time making the Flash stuff blend in better with the sprite work. It all looks REALLY out of place, the most egregious examples being Robotnik's fortress and the rocket facility. I think you could have taken the time to actually shade things and make it look more organic and believable, but they stick out like a sore thumb and draw me out of watching the flash.
-a TINY bit more time working on them hands. Lol, I know they're tough to draw, but they still aren't quite there. Keep practicing!
This is not REALLY a criticism, but:
Yeah, this is pretty J-Poppy, even with Amy doing the whole "Shrine Maiden" whatever the hell. It ain't really quite my thing (at least not any more after seeing it a lot), but I am sure it'll work for plenty of people. However, I did lol at the picture Sonic was carrying of Amy being kidnapped. I wonder: Who took the picture, and why didn't he do anything about it? Ha ha
So I give you a 7 (which if you look through my flash reviews, is pretty high up there) for the overall effort put into this (which is commendable), and take 3 points off for the rough spots that detract from the viewing: the out-of-place flash, the kinda questionable tween session towards the middle (???), and the overall flavor, which isn't totally my thing.
Still, a very good job, and I expect a generally positive reaction.