I just started, and I am captivated. I have however run into issues.
I have to say that the special stages are quite difficult. The Sonic 2 Special Stages were difficult as is, but the controls at least felt natural and responsive. I feel like the jumps are the biggest issue here. They feel odd and almost too rapidly bouncy. It's going to take some time to get used to them. I do have
Secondly, it is quite labyrinthine. To me, a sonic game is supposed to feel like it is meant to be either blazed though or explored. In sonic 2, 3 and S&K, I got the sense that if I wanted to, I could choose either path and still have fun. Here, there are many barriers to the pathway, and backtracking is at best, required and at times, inescapable due to the construction of some of the paths. That doesn't make it bad per se, but it makes it a bit difficult for those who want to get a quick tour and then return to savor it again afterwards, or to speedrun it.
My last concern thus far involves a potentially game breaking bug I encountered. I had, to my amazement, just gotten a chaos emerald in Rusty Ruins Zone and immediately got killed as I had no rings. Unfortunately the last star post I passed was underwater, and astonishingly it didn't even allow me to get past the opening screen of the reload. I heard the immediate dinging of near-drowning warning alarm and the second the game allowed me to control Sonic, I drowned. This proceeded to happen over and over until I lost the lives I had left. I will give you props as to the save function allowing me to retain the stage and chaos emeralds collected. But the fact that the problem is there is frankly ridiculous and I'm astonished that nobody else had discovered it. I looked through the bugfix updates and didn't notice it mentioned, and I believe I have the most recent version, in addition to the "Saturn DLC".
On the other hand, I am quite pleased with Sonic's handling in the regular zones. While sometimes I feel like there are hiccups, it feels close to natural for a Sonic game. My only issue there is that I'm not quite used to the millisecond delay of getting sonic to duck. In addition, I'm pleasantly surprised that you opted to include the Peel-Out function from Sonic CD.
Next, I'm very interested in the powerups you've included, especially the lock-on shield. In addition, retaining Sonic's "spark" move from Sonic 3 and Knuckles makes me extremely happy.
Further, The tilesets and graphical design are breathtaking. They feel practically in line with the aesthetic of the Genesis titles. I'm especially impressed with the boss designs for Robotnik's machines (Am I correct in that they're kitbashes? They're some of the best ones I've ever seen) and that you included Nack/Fang as a miniboss. Absolutely floored. I'm looking forward to completing the game.
While the game isn't perfect and in my opinion needs a little bit of functional polish, it's a great achievement in what fangames can present. It'll definitely hold my interest until a particular Megaman fangame completes development.
Addendum:
I've been playing and I have grown quite frustrated with the pace and enemy placement in some sections of the game. I've escaped drowning since I posted the review, but I've accidentally started a new game over the one where I was finally on Puppet Panic zone. I've lost the chaos emeralds as a result, and all my progress.
In addition, I've found myself becoming infuriated at how the bosses and enemies have been programmed and in some cases located. Many times I've found my forward momentum completely halted, if not by the slow halting progression of the labyrinthine level construction, then by being hit by an unforeseen enemy, and points at which I've expected to be able to hit a boss only to be met with either being forced to the other corner of the arena or straight up taking damage. They're excessively difficult. I've also found the amount of shield monitors to be quite sparse, and oftentimes I've lost it within 15 seconds or less because of a misjudgment or enemy placement.
Speaking of labyrinthine level construction, Gene Gadget, while a beautiful level, took me a frankly unfair amount of time to get used to. Having to control your progression through multi-path pipes, especially in that near-last section was rage-inducing and I timed out. The whole game features levels that take much longer than 4 minutes to get through going practically as fast as one can, and when you add a mechanic that slows your progression as much as that, it's painful.
Overall, while I appreciate and applaud your attempt to be as faithful as possible to the theming and level construction of 3D Blast, I dont feel like it works in 2D.