The gameplay itself is brilliant. Each of the 9 stages is a throwback to one stage out of Sonic's massive video game carrier, excluding spinoffs. Some of the highlights include Chemical Plant, Sky Sanctuary, and surprisingly even the more modern sonic stages are done well.
Each "stage" is actually two separate stages, one for Classic Sonic, and one for the Modern Sonic.
In Classic Sonic, you hold a button to charge a boost while standing in place, and release to blast with speed into things. You are also limited to 2 dimensional movement. These stages often are made of platforming segments that also serve as a tribute to the old games, many times sharing the layouts of the older games. In the later stages, these become some pretty diabolical all new platforming levels since the later games source is all in 3d gameplay. My only real complaint here is that holding the button down to charge up a boost, while easier to control, just doesn't feel quite right after the true originals had you mash the button repeatedly to "shift" up in speeds.
Modern Sonic seems a bit weird unless you've played Colors. Most of his stages, you'll be running straight ahead, using some slick redone Colors type controls to react to what's coming up. There are a lot of action sequences in the 3d levels where something is chasing you or the stage is falling apart and you need to move fast, using your Colors "Instant Boost Bar" in the correct spots or right timings to earn some shortcut upper routes.
The stage design itself is awesome. The stages are designed with "chokepoints" where all paths rejoin, but in between the path often splits or is divided into classic "upper, middle, and lower" paths. The harder paths are also shorter and more rewarding for score, with the lower ones always being much slower then the upper routes, in classic sonic stage design.
The keyboard controls are adequate, if a bit weird compared to other games controls. After playing the first stage on keyboard I quickly switched to a gamepad. I'm happy to report that the PC version supports any gamepad thanks to it's vastly useful control configuration options. The prompts in game will still show the 360 commands, and there are two LR triggers that are used in the game, but once I got used to what prompt meant what on the controller the game played just fine. I used a Playstation 2 controller with a usb adapter, and can happily report that controlling sonic with the controller is one of my smoothest control experiences.
The game's main storyline is a bit short, and can be completed in a matter of hours. Like Colors, there are lots of extra things to do afterwards that help lengthen the playtime. The story is short but sweet and to the point, and thankfully not full of the nonsensical fluff that plagued a lot of the dialog in the older 3d sonic cutscenes.
The boss fights, while few, are all pretty well designed. The rival fights for some of the emeralds also are pretty good at referencing the old material(like the Sonic CD Metal Sonic battle) while keeping it fresh and more playable to match with the rest of Generations.
I must say that the final boss fight mechanic was a bit of a minor letdown, but it's also a throwback to the classic boss fights of the old 2d sonics(complete with ring countdown), so in the end I guess it does work pretty well to sum up the game.
The thing that makes this game shine out as far as replay value goes is the amount of challenge stages, all of which have been designed as completely new stages. To finish the storyline you only need to pass on 1 of each world's challenge levels, and each world has anywhere from 5 to 7 of these extra stages in it. Many of them are full blown new levels with a focused new twist on playing the game, so 100% completion players will be here for quite some time after the credits roll. These challenge stages give out prizes too, like concept artwork, music tracks from other sonic games(which you can then select to play on any stage as the background music), and even some new moves or mods for Sonic in the shop that change the way he moves a bit.
In addition to just completing the challenge levels, every stage, challenge level or not, has set times to beat to "S rank" them. The only way to restore the map world to it's true brillance is to obtain these S ranks, though even a D rank(barely passing the level) will earn you the loot. As you can imagine, some of these rankings are going to take a lot of practice and several runs through the level to obtain.
If that wasn't enough, the Sonic Colors "5 Red Rings" are also back. Each of the main stages has them, some are in your face, some as well hidden as the old Giant Bonus Rings in the original sonics. Getting to them isn't enough, in true Sonic Colors fashion, if you die after collecting one without getting to a checkpoint, it's gone for good and you have to get it again.
You're getting a LOT of sonic for your bucks with Sonic Generations, and all of it has been refined and perfected by all the stuff SEGA's learned from their previous games on what works and what doesn't work. There are very few minor rusty parts of the gameplay, the rest is smooth, fast, beautiful, and works, just like Sonic should be.
There is one major negative thing worth noting about the game, for those who are planning on getting it. The PC version only has 1 save file. This is very strange, because the other ports of the games have always offered 3(Sonic 3D Blast, Sonic CD) or even 6 (in the case of Sonic&Knuckles+Sonic3). The last console game, Sonic Colors, also went out of it's way to have 6 save slots. So it's kind of weird that Sonic Generations only uses 1.
I'm guessing on the consoles, it would have separate saved data for each "player profile", but it still seems a bit lazy not to at least let players have the usual 3 slot console port saved game setup. No big deal if you are playing by yourself, but it's a big issue if you want to start a new game for your sibling to play through the game fresh. Now, every cutscene can be replayed in the gallery, but it's still just not the same as playing it and watching it unfold from a fresh start.
Overall though, this is a sonic game not to be missed. I certainly don't regret purchasing it, and if you liked even one sonic game in the past, you probably won't either.
In the end, the whole game is like a massive love letter from SEGA to it's Sonic fans, and it's full of the sweet candy moments that make Sonic great.