Not quite there yet. Too much re-hashing, and using old ideas, not enough addition to the series.
Now, back in the day, this was understandable. I mean, Sonic 2 had levels cut from it, and Sonic 3 was going to have the same thing done to it.
Sonic 3 needed the 2nd cart. Sonic 2 should have had it. Sonic 4, under no circumstance, should have been divided. There is no excuse for that in this day, and no one really wants their games divided, and, on top of that, SEGA released 4 stages (just four) with three acts.
And, if I'm wrong, Someone please tell the other game companies to start releasing their games in episodes.
I would have rather had 6 stages with two acts, which is, still too short, by the way.
Another issue with the game is the bad physics. Sonic on Genesis had good physics, why can't this game? If you do a running jump, and press the opposite direction on the directional pad, it feels like hitting a brick wall. He stops, and looses his momentum. It really feels and handles like a fan-made internet game.
I'm not saying the physics need to be real, but, this is simple stuff.
1: Sonic should keep momentum. The game's supposed to be about speed (to a degree) why doesn't he retain momentum. It slows the game down and makes it feel sluggish. A Sonic game that feels sluggish isn't good.
2: Sonic's homing attack works too low to the ground. This is an issue because if you want to spindash, and try it too soon, while in air, you're going to air dash towards the ground.
3: Sonic doesn't fall in an attack state, and, doesn't keep momentum, so, he's going to fall slowly.
4: Try walking up a ramp. He will stroll in place... This NEVER happened in the Genesis games. You either strolled up the ramp, or fell down it, not stayed in place.
Now, if these four minor problems had been fixed, the game would feel so much better. But, it would still be far from perfect. It may seem small, yes, when it's one minor small issue, but, if you have enough cups, you can hold a gallon of water. One issue wouldn't have been so bad, but, when you've got all four, well, that stuff adds up.
Remembering what Sonic did in previous games...
From Sonic 1-2:
Spindash (New Attack)
Tails (new character)
Two players
Spike difficulty fix. (Minor issue, btw)
More stages overall.
Addition of Super Sonic
Special Stages qualifications altered
New Special Stages.
New badniks, with some old ones.
And, from Sonic 2-3
Insta-shield (new attack)
Tails can fly manually.
Knuckles (new character)
More stages. (14, including S&K)
Elemental Shields
New special stages
Bonus stages
References to the previous Sonic Zones and boss battles, without over-doing it.
Stages had lead-ins to the others.
One of the only Genesis games with a SAVE feature.
New Badniks, with a few old ones
Hyper Sonic
So, with Sonic 4, you would expect it to have everything Sonic 3 had plus more.
Well, there's only 4 stages, the other characters have been removed, and there is no new character addition (although given SEGA's record, maybe no new character is a good idea).
There's no insta-shield, Bonus stages are completely gone, as the checkpoints don't do anything, other than resetting you from that spot.
The special stages, Zones, and bosses are all rehashed from previous titles. And, Robotnik's theme is taken from his SCRAPPED Sonic 3D Blast theme.
If a theme got scrapped way back then, what makes anyone think it would be a good theme now?...
So, with all this said, the game doesn't have the level of "addition" sonic 3 had over 2. That and the lack of originality, bad physics, bad synthesized music, and nothing new to the game... Why play it...
If you want to play:
Green Hill Zone
Casino Night Zone
Labyrinth Zone
Metropolis Zone
Sonic 1, and 2 await.
The only good thing about this game is the graphics. Personally, it wouldn't matter to me if the game were in 3D, or 2D, or the 2D graphics that resembles 3D, like what they did.
I really can't even suggest this game to people.. Obviously some will like it. Others will hate it.... Just try the demo, and try to be optimistic about it. Maybe you wont be as let down as I was.