SERIOUSLY, SPOILERS.
Technical stuff first:
Is it as good as the Genesis games? Yes.
Is it better than Sonic 3 & Knuckles? No.
Rating? 10/10
Now for the gushing. 99.9% of this game is fantastic. While most of the zones are old they are remixed so much that they may as well be new. Act 1 is usually a condensed version of the original with Act 2 being brand new concepts. Throughout it all the game combines concepts of various zones. Flying Battery mixes with Wing Fortress, Stardust Speedway mixes with Marble Garden, Lava Reef with Quartz Quadrant, Oil Ocean with Sandopolis, etc. I love it all.
My favorite new zone: Press Garden. Specifically Act 2.
Bosses are a mixed bag. Some are awesome like the Mirage Saloon Hard Boiled Heavy or Metallic Madness Robotnik. Others can be irritating like Oil Ocean Zone Robotnik (a mix of the original Sonic 2 boss and Lava Reef mini) or Lava Reef Hard Boiled Heavy. And sad to say, the Super Sonic boss at the end was awful. A closed arena, hard to see what's going on, an attack that can actually harm you, and a worthless "boost" that does nothing but use up some rings. It was horribly disappointing. Then there are brilliant surprises like Mean Bean Machine at the end of Chemical Plant.
Music was absolutely flawless. Remixes and new tracks alike. Nothing bad in the entire soundtrack.
Knuckles' run was slightly disappointing. It felt more like a "Knuckles in Sonic 2" run more than anything else. There were barely any divergent paths exclusive to him (that I was able to discover anyway) and the only unique zone was Mirage Saloon Act 1. However in a way I enjoyed this run more since his "path" actually skips a couple bosses I didn't care for.
The Special Stages are probably my favorite in the series now. Like an enjoyable version of Sonic CD seasoned with a bit from Sonic 1 and Sonic 3 & Knuckles. They also have that nice Sega Saturn polygonal goodness. Not to mention the return of Super Knuckles and Super Tails! Super Sonic and Knuckles style is actually that of their hyper forms from S3&K, but lacking the additional abilities (flashing and ground shaking) and Tails ditched the Flikies sadly. It's also dumb that going super wasn't mapped to another button. The game essentially only uses the D-Pad and a button to jump. What the hell? I hope that gets patched in later.
Blue Sphere has been perfected. I had actually grown tired of them over the years from playing so often, but it runs so silky smooth now it's almost like playing something completely different. There were no flaws outside of what I would consider a couple poorly designed courses. All failures were 100% on me.
There are so many little details that I'd like to share, but I don't want to cause this post to ramble on. Simply put, this is the definition of a love letter to fans. I know at least one other has played this here. Any others?