Immediately you will have all of the classic Genesis games unlocked from the start. Along with Sonic the Hedgehog, Sonic 2, Sonic 3 and Sonic & Knuckles, there is also Sonic Spinball, Sonic 3D Blast, and Dr. Robotnik's Mean Machine. With a variety of both the main trilogy and the spin offs, there's plenty to offer. There are also 3 other Genesis games that aren't Sonic-related but you can unlock them as well (Comix Zone, The Ooze and Ristar). These games play as great as their Genesis counterparts and are perfectly intact, glitches untouched making this a perfect emulation. To make things easier for you, there is also now a save feature for you to use as well.
In addition to this large variety of games, you can also unlock other variations of them such as playing Blue Sphere, "Sonic 3 & Knuckles" or Sonic 2 with the S&K lock on (allowing you to play as Knuckles in Sonic 2). Unlocking them, however, is the annoying part. To play these modes, you have to start the other games a certain amount of times. For instance, in order to unlock Blue Sphere, you actually have to start up the original Sonic the Hedgehog 50 times and to unlock the other games, you have to start up the remaining Genesis games a certain amount of times as well. This is just pain, why couldn't we just be asked to beat a certain game to unlock them instead? The only difference are the extra Genesis games which just require you to play Sonic Heroes on your Xbox or something. Don't have or want SH? Too bad.
Oh, and there are also a large collection of Game Gear games like Sonic the Hedgehog, Sonic Chaos, Sonic Drift, Robotnik's Mean Bean Machine, Sonic Labyrinth and Sonic Spinball. Some of these games are decent but others will make you feel "why did they bother?" (like the inferior ports or Labyrinth). Well, it is interesting to see what I missed out on.
Other than a large collection of classic games, there are some early issues of the Sonic comics by Archie to look through and concept art, from Sonic 1 to Sonic Heroes. There are also in game instruction manuals taken from the classic Genesis booklets themselves. These extras are nice, especially the first few issues of the Archie series.
Although it may seem like this collection was outdone by other collections, namely Sonic's Ultimate Genesis Collection, this is a decent collection of Sonic's Genesis era (although at least it has Sonic 3 & Knuckles). It may be lacking games Sonic CD or Knuckles Chaotix, but this collection will take you back to when Sonic was on top of the charts and giving you some of the best platforming experience to date. If you are looking for a good collection of Sonic games or if you want to see what made Sonic so great in the first place, you really can't go wrong with Sonic Mega Collection Plus.