And I'm glad I did. Now I don't intend to make this a rage post, so don't jump down my throats. And this is all coming from personal opinion by reading tons of posts and reviews. Maybe I'll D/L the demo if they have one on Steam and try it out.
Anywho, I heard that Super Sonic's physics weren't fixed? To me this is an absolute joke and disgrace. As an aspiring game designer, that would be one of the things I made sure was fixed right away. There is no excuse for inconsistency in game play.
As for the physics? I don't understand why they wouldn't be near genesis replications. I'm sure everybody has heard about this by now, but it is Sonic 4...it should follow Sonic 3's physics and improve upon them (this is an obvious step backward).
The unoriginality also doesn't sit well with me. The only zones that feel...uhh...new for me, or at least acceptable are Sylvania Castle and Oil Desert when it comes to not rehashing the same themes (sure, say all you want about White Park, but to me, it's just Ice Cap with a really linear level squished between the two).
See, as far as I'm concerned, sequels are intended to take what made the predecessor good, and build upon it with new and innovative ideas. To me, Sonic 4 is just one giant nostalgia trip that fails on the nostalgia... -.- This doesn't make sense to me. O_O
I will give it credit where credit is due, however. Co-op features is a great idea, time trial (that's in, from what I've heard?) is also a great idea. Metal Sonic returning is nice, and the music in some places is good (I liked Oil Desert Act 1's music, and Metal Sonic's boss music, both awesome).
Other than that, if this game was released on the Genesis, it would fall into "bad sonic" category very quickly.
I welcome 2D sonic games, I think that's where Sonic needs to go, but the execution needs to be better.
Please take what I say with a grain of salt. I'm just sharing my opinion from observing the Beta videos and the final gameplay (which I heard is pretty much identical?).

