HowLongToBeat.com lists this game as taking five and a half hours to beat. Compare that to my playtime. I've gotten to the point where I can beat most levels in two or three minutes and I still keep going back to them. It's not a hugely challenging game but the experience of just running through the levels is delightful and relaxing while still demanding your attention to successfully complete. Shaving off seconds of your best time or just messing around and exploring the alternate paths makes for a fun distraction of an evening.
I generally go back and replay Sonic 3 & Knuckles at least once a year. I love that game and it was by far my favourite in the franchise. I say was because Generations is now a very, very close second. Only the final boss - which, in contrast to the rest of the game, is a trainwreck - holds it back from the top spot.
The nine levels from the game are taken from previous games. Revisiting some favourites from the old games is lovely, but these levels are great by themselves apart from the nostalgia factor. Each level is split into two acts, in the first you play as "Classic Sonic" who is styled after Sonic's sprites from the very first game. He handles very much like the Mega Drive games with only minor differences in physics (though his spin dash has been upgraded to the point of being almost hilariously powerful and fast) and really captures the old feel in a way that Sonic 4 absolutely did not.
Also unlike Sonic 4 the game makes full use of its 3D environment while still playing entirely in 2D. The background scenery is gorgeous and used in ways that would not be possible in a fully 2D game.
The 3D levels play like an amazing mix of standard 3D platformer, racing game and rhythm game. It moves much faster than the 2D sections but the forward facing camera allows you to see what's coming up ahead and react accordingly. This half of the game is much more dynamic with far more buttons on the controller being used as Sonic runs, jumps, boosts, slides, stomps, sidesteps and drifts through the levels.
The levels, both 2D and 3D, are much more designed than those of the Mega Drive games. There is less general exploration and open areas than in the old games. This is not necessarily a bad thing. The rule of high route = fast and low route = slow is enforced strongly throughout the game. Failure to hit a jump correctly does not (usually) lead straight to a bottomless pit but instead to a lower path. These diverging paths periodically meet back up for the levels set pieces which provide some spectacle for the player before allows them to try and get to the top route again. It's a clear system that gives the player a goal on replays: Learn the top paths and get them down to a tee.
There's also a variety of side missions for each level. Only 9 out of 90 need to be completed in order to beat the game, making them a nice mostly optional challenge for those who want to get a bit more out of it.
The game features 7 boss battles which are all quite varied but all, apart from the atrocious final boss, are great fun.
As for the story, there's thankfully not much of one.
There's also some nice extra level mods for this game both released and on the horizon.
I foresee myself replaying this game for a long time to come.