Okay, the game itself was good. The platformer part was really great. All the zones here were neatly transfered from 3D to 2D. That's great... Now about minuses. There are some of them unfortunately.
First, the screen resolution. It's not THAT bad when I go for the full screen the image it doesn't get very stretched, but it still somewhat ruins a really good picture.
Second, it was probably only my problem but when I uploaded Saturn DLC music the game crashed for some reason. Only once though. The same crash never happened again. So maybe my computer just had some weird brainfart at that moment.
Third, and my main complaint, the special stages... Sorry, but they are bad. I never liked Sonic 2 Special Stages in the first place, but here... The controls are somehow even worse. The things like rings and bombs appear way too late, with no time to react to their appearance at all. In fact, original Sonic 2 special stages had some specific rules to the rings appearance in them. Like for example, when the tube turns rings and bombs appear only on the side which is visible to the player and NOT on the side where you can't see nothing.
Now, I am not saying that the game is bad becasue of it. No, of course not. But I will say, that these special stages really ruin the experience for some players who hate those Tube Special Stages. And yeah, Saturn Sonic 3D Blast had those stages too, BUT they were easier since they were in 3D and gave time for reaction, and again they followed the rules in placing rings and bombs during the course.
So yeah, that's my opinion on your game. But to not end on the bad note I gotta say that was a really smooth ending with Robotnik\Eggman finding the Chaos inscriptions on the walls of the Temple. Really nice! Especially considering that according to Jap. canon all the events of Classic Sonic are taking place not on Mobius but on our Earth, so it's logical that after 3D Blast Adventure Series starts)))