The 3DS version of Sonic Generations is more of a miss than a hit.
The story is near enough the same as it was in the console version of Generations, with the only real difference being that Sonic's friends (bar from Tails) do not make an appearance. On Sonic's birthday of all days, a mysterious creature known as the Time Eater seems to suck in the world around him, kidnapping Tails and turning everything into a lifeless white world. During Sonic's quest to restore time and the world to normal, he encounters his younger self in the guise of classic Sonic from the 16-bit era. The story is told through text-boxes which while not visually impressive does manage to get story across just fine, and while there is little in the way of character development or humour, the story really only serves to explain why there are two Sonics running around saving the world.
As far as the gameplay goes, it's certainly playable. Like in the console version, you can take control of both versions of Sonic, both of whom will be needed to clear a level. The cute, podgy Classic Sonic initially plays much like you would expect him to; his gameplay resembles that found in the classic Mega Drive games in that Sonic will only be able to roll, jump and spindash. However, this simplistic set-up soon changes after beating the first era, when Classic Sonic is granted the homing attack. Sadly, unlike the console version where you are given the option to unlock it and add it to Classic Sonic's moveset if you wish to, it is automatically added to Sonic's moveset here and there are many points from the second era onwards where the level designs seem purposefully set up to try and encourage you to use the homing attack. It doesn't seem like an offensive addition, but the biggest problem is that it draws more similarities between Classic and Modern Sonic, making playing as two different versions of Sonic seem pointless. I will, however, say in Classic Sonic's defence that the physics seem much closer to the classic Sonic games than even the console version of Generations, which is a nice touch.
As for Modern Sonic, instead of having 3D sections and playing like he did in the console version, he plays much like he did in Sonic Rush, leaning towards fast-paced 2D gameplay. He can use the homing attack from the get-go, can boost and wall-jump in addition to obviously being able to run and jump. While the boost certainly does make Modern Sonic's levels a tad easier than Classic Sonic's levels, there are points where you do need to do some platforming so you cannot simply boost to win. The biggest problem is, Modern Sonic and Classic Sonic both play in 2D and while Modern Sonic does have a few extra moves up his sleeve, the gameplay styles really aren't different enough to justify having two versions of Sonic playable.
Much like the console version, Sonic Generations has a series of levels from previous Sonic games, from the Classic Era all the way up to the Modern Era. However, bar from the exception of Green Hill Zone, the levels that are used in this game are completely different to the console version of the game. Instead of Chemical Plant representing Sonic 2, you have Casino Night, while Mushroom Hill represents Sonic 3 & Knuckles instead of Sky Sanctuary. The level choices themselves seem fine, though it would have been nice to have had levels representing the handheld series of Sonic games. True, Water Palace from Sonic Rush and Tropical Resort from Sonic Colours make a comeback in this game, but there is nothing representing the Game Gear games nor the excellent Advance trilogy, which I found disappointing. It's also worth noting that there are only seven levels in this game compared to the console version's nine levels.
The biggest issue with the levels, however, is their design. Classic Sonic's three Classic Era levels feature the exact same level design that was found in the original Mega Drive games with no difference whatsoever. While these stages were fun in their original games, it's disappointing that this approach was taken when you consider that the console version had different designs for existing levels, making them seem fresh. Sadly, even Modern Sonic suffers this copy and paste level design approach when it comes to the Modern Era of levels. Thankfully the rest of the levels seem to hold a lot more originality to them, but that doesn't necessarily mean their design is better; some levels have points where you are simply pressing nothing but right on your directional pad, and a lot of the time, none of the levels strike me as memorable.
After completing both acts of each stage (one act for each Sonic), you have the option to access a special stage, a feature sadly missed in the console version. The special stages are designed after the Sonic Heroes special stages, featuring you running down a pipe chasing down a Chaos Emerald, collecting coloured orbs to boost your speed and avoiding bombs. The plus side to these special stages is that they control a lot better than the Sonic Heroes ones ever did and as a result, it doesn't feel like a tedious struggle collecting the emeralds. However, at the same time, collecting the Chaos Emeralds sometimes feels too easy, due to the fact that you can access the special stage any time you want after beating two acts of a stage. Previous Sonic games saw you needing to fulfil certain requirements (such as collecting 50 rings or finding hidden giant rings) in order to access a special stage and simply being able to access the special stage whenever you want doesn't reward gamers for exploration or exceptional skills.
Another addition you can expect from the 3DS version would be boss battles, of which there once again are seven. Unlike the console version, rival battles have become compulsory if you want to reach the Era's true boss. It's worth noting that the rival battles are also nowhere near as unique or as enjoyable as the console version's of the bosses. For all three rival 'battles', you simply have to race against your rival in a portion of a level you've already completed. They are very reminiscent of the Metal Sonic boss fight way back in Sonic CD, and if you enjoyed that fight, then you may well enjoy these 'fights'. I, however, found them to be dull and very easy.
The main bosses, while a fair bit more enjoyable to fight, sadly also suffer from being as uninteresting as the races, and in some cases, are also a touch on the easy side. The only boss in this game, in fact, that I feel is better than the console version's fight, is the last boss and that is due to the fact that some layer of strategy is involved.
Outside of the main story mode, which is incredibly short and only took me approximately a measly two hours to beat, there is the addition of a mission mode in an attempt to bulk out the package. However, unlike the console version's missions, most of the missions here aren't very interesting and nothing compels me to try and beat them due to the fact that they simply lack variety. It's also worth noting that this game does have multiplayer as well and while it's an okay addition, it does nothing to save the game from it's own mediocrity.
Overall, Sonic Generations for the 3DS isn't necessarily a bad game but it's certainly a disappointing addition to the line-up of handheld Sonic games. Graphically it's a pretty decent looking game and the 3D works well, and the music, for the most part, is really well-done and nice to listen to. But taking into consideration it's ridiculously short length (considering the price of most new 3DS games) and mediocre gameplay, I would only recommend trying this one out if you are either a die-hard Sonic fan or a fan of 2D platformers.