This hack is a very interesting version of Sonic the Hedgehog 1. In this game, you can play as one of any three of Sonic, Shadow or Mighty. The game also states that Tails and Knuckles are playable too, but since I've only just started playing this game, I can't figure out how to unlock them. So your choice is slightly reduced. Nonetheless, its still a great hack to play, very challenging, more so than Sonic 1. The zones are different compared to Sonic 1, in that there's a different background, different color, different music and a different path that the character must follow to complete the level. The levels are, however, made up from features from the Sonic 1 version, so for example, the fourth zone of the game (Misty Maze Zone) is made from features contained within the Labyrinth Zone of Sonic 1. Also, a character will have additional abilities, such as speed boost or homing attack, and the collectibles are from the Sonic 3 and Knuckles game, giving the character additional useful items such as the lightning shield. These extra abilities and collectibles do not appear in Sonic 1. The level bosses are unchanged, except they appear in a different area of a level, but the final boss level is much harder compared to that of Sonic 1. The special stages are slightly different. Aside from the fact a character must have six emeralds from each stage to transform into super formation (again not possible in Sonic 1), the bumpers have changed completely. Instead of bumpers from Sonic 1, the bumpers are from Sonic CD, so the sound will also be different too. Also, there are lives (not in monitor form) to collect at a certain part of the stage. Personally, this game is wholly a massive upgrade in comparison to Sonic 1, and the added features and character abilities are definitely a reason why this game is addictive.  

Have you tried 4.0? it's 3.0 with more features, such as Knuckles and Tails, secret stages, and an new Sunny shores and a new Dark fortress! (but no, Knuckles is not in this game or Tails either) But still a nice review.