This is the real Sonic
Forget anything that has come out since the mid 90's. Except for maybe a couple competent GB titles and a DS one or two I never played, Sonic had strayed from his core 2D gameplay to confusing badly controlled 3D versions that never really captured the true Sonic feel. Sonic 4, now released 16 years after the 3rd, follows directly in evolution of the series as if nothing has come out since, never mind the million color HD graphics.
I'd get into more detail, but it may be moot if you have never played the original Sonics. This may not be what one would expect from contemporary action 2D platformers. Sonic is sluggish to get up to speed, has no weapons but a dash attack, and once Sonic is blazing fast everything is going by in an overwhelming blur that God knows where he is going to end up. To fans of the original series, this comes as a welcome balance where all this fits into an invigorating frenetic pace that allows quick reflexes and constant decision making on which path to take Sonic to find as many rings, power ups, and collect points from bopping enemies as one can. Occasionally sonic must stop and puzzle his way through a level with careful timed jumps or button presses. At times I have gotten this "I have done this before" feeling as the levels use some of the same mechanics as the earlier 2D sonics, but still there are refreshing new moments that take advantage of hardware newer than a Genesis.
Sonic 4 does not mess up the original balance of the gameplay. Really its as if nothing has changed in the past 15 or so years. In fact some things from Sonic 2-3 have been taken out to streamline gameplay down to the core mechanics of what made Sonic fun back in the day. The only new maneuver is an in air dash and the homing attack. While mid air Sonic can do a quick jump one direction ot the other, or he locks on to the nearest object and dashes straight towards it. At first this took getting used to as it is easy to over-use by prematurely shortening Sonic's jumps and making one miss platforms, but once one gets a feel for it is a vital part of the gameplay. Some level paths require it to hit a series of enemies or jumpers in sequence to get to that hard to reach extra life, and its quite handy in boss fights.
I'll wrap this up: If you EVER liked the original Sonics then you WILL like this one. It is not trying to fix what was originally not broken. For those worried if it is worth the $15 dollar price I say it is if you are the type to replay a level to explore different paths, get a higher score for the leader board, and collect all the chaos emeralds. If you are the type to run through it only once then it will feel rather short, although fun while it lasts.