The; no, by FAR the worst old school 2-D Sonic game.
First things first, S&K wasn't even considered a game at the time it was released. It was an 'expansion pack' to Sonic 2 and Sonic 3, or, to put it in other words, a quick cash-in attempt by SEGA. It is supposed to be the direct continuation of Sonic 3, but instead of cramming it all in one game, SEGA probably decided to rip fans off more. By having both Sonic 3 and S&K you could use this 'lock-on technology' gimmick (this isn't 1994 though, so in the year 2011 that means you have to own a Wii and download both of the games) and get the full game entitled "Sonic 3 and Knuckles", which is a pretty big and solid platformer on its own - but this review is about S&K, and S&K alone.
Does S&K meet up with the first couple of Sonic games? Short answer - no. It does try very hard though, and you have to give credit for it; absolutely nothing new is introduced to the series. You run from point A to point B, finish the first act, proceed to the second act, clear zone's boss, onwards to the next zone. As a sequel, some similarity with its predecessors would be understandable, but HOLY CRAP! You guys are on your 4TH installment of Sonic (7th if you count the Game Gear releases), you cannot just make another clone and expect us to pay for something we have played over and over again! It feels and plays exactly like Sonic 2, which feels and plays exactly as Sonic 1. They didn't even bother to spice graphics up a bit; every game feels so much like the last game, to the point that you think Sonic Team was just making one hell of a big Sonic game, then decided to cut it down to parts. Sadly, SEGA got what SEGA expected: people do indeed love wasting their money.
So, as said, nothing in this game is new. Oh wait, at least they bothered to make semi-new zones (and by semi-new I mean they look very much alike to zones of older games. Mushroom Hill Zone; Green Hill or Angel Island, anyone? Flying Battery Zone; Wing Fortress, anyone? Not to mention Lava Reef is Marble Zone meets Labyrinth - I could go on about every zone in the game but you get the point). Oh, plus Sonic's eyes are more white and less black than in older versions. Oh yeah, and there's Knuckles. Knuckles is the first (technically the second) utterly useless character of many to come in later years. He was first introduced in Sonic 3 and serves pretty much the role of the idiotic douche in the series. Knuckles is playable, yet there is very little change in gaming; the boss battle is slightly different, plus Knuckles can glide and climb walls. It sounds good on paper, but very little of this 'gliding and climbing' is used to your advantage, unless you want to explore the level for secret stuff, something I never bothered to do.
So the graphics are the same, the music consists yet again of annoying little catchy tunes (Mushroom Hill's music sounds like something a drunken bard would play, however), the gameplay is same old; but at least it wouldn't matter as much, if S&K was as enjoyable as the first two games, right? Well, the fun factor is where S&K completely falls flat. The formula is horribly linear, as always (Beat 6 or 7 Zones consisting of 1 or 2 Acts, collect hidden Chaos Emeralds to unlock an extra final stage), but that is not the problem. The execution of it is atrocious; it's like Sonic Team managed to suck out the fun and leave behind this mess of bungled platforming levels, whose design will haunt you in your dreams.
Let's get this clearer; you begin with Mushroom Hill Zone. Classic-themed, simple, easy for beginners and it plays smoothly; a spring propels you up in the air, another spring sets you speeding away, a loop-de-loop here and there, a few baddies on your way. There are some parts where you have to move cautiously and do a specific little task that slows you down a bit (like the lifting machine), but overall you like what you see, the game is pretty speedy but not a cakewalk, it takes about 2 to 3 minutes to finish each act - all is well. Come Flying Battery Zone, you have to forget everything you experienced in Mushroom Hill. The difficulty rise is uneven, as it feels just like you stepped into one of the game's final zones. Oh yes, S&K is as hard as nails, but for all the wrong reasons: overly long acts that by the 6th minute they have you screaming in your mind "When does this end??", a complete lack of a save system and impossible-to-get continues (that means that after three tries or a couple more, if you manage to find a life box, you get the Game Over screen and thus are sent back to the beginning of the game, even if you were one step before beating the crap out of the Final Boss. Fun, right?), little things that make you lose a life in a heartbeat (i.e. #1: if you jump into one of those huge spiraling tubes, sometimes you get in, sometimes you fall out and meet your doom. i.e. #2: when your character gets hit, your rings fly into every direction, but for some reason the only rings you can get back are those that are tumbling away in an unreachable direction, 'cause the ones closest to you just go through you and disappear in a matter of milliseconds). Have you ever had a "Time Over" message appear when you were regularly speeding through a Sonic level? I got my first one at the second act of Sandopolis, because the developers simply do not know when to STOP. An act is meant to be fun, rather short but challenging; there were a couple of lengthy acts in previous Sonic games, that took roughly 5 minutes to beat, but with the exception of Mushroom Hill here, EVERY other act is 5 minutes at LEAST. And don't think you'll be speeding your way through any of those 5-8 minutes, oh no, Sonic Team managed to screw Sonic's trademark too. Instead, you'll find yourself pushing a block to temporarily open a door, pulling a lever to alter something in the stage, pushing a button here and then another one there to raise a ledge and open a door so you can proceed through. Is this a Zelda hybrid? Am I supposed to look around the level for a Dungeon Map and a Compass? Then there's the occasional loop-de-loop and booster, to remind you that you're playing a Sonic game, and that it's speedy at its core.
Things just get worse when the level design comes in. Naturally, I would be able to endure a 7-minute borefest if the design was at least decent and if something could get me hooked most of the time (i.e. something that causes a change of environment, like the first Act of Sonic 3). S&K's level design is as broken and as tedious as a Sonic game can get; not only are the stages unnaturally huge, which makes you actually race against the clock ticking 9'59', they follow the same pattern over and over and over again. Raising the sand level in a stage can be fun for a couple of minutes, but it just gets painful the third time you have to do it; neither is swinging from a rope while sliding down a wall for the eleventh time fun. I often get the feeling that I'm trapped in some nightmare of a zone, because I keep doing the same things over and over again; break this wall, fall down below, push switch, door opens, run for a couple of seconds, then repeat. This isn't really innovation at its finest, and having to go through that for about 8 minutes for each stage would make anyone shut off their console.
All of the above, with a bit of trial-and-error added, make the game virtually unplayable. From the second Zone and on, fun gives its place to frustration, so unless you like struggling for a couple of Zones and then going back all over again, don't even think about giving your 800 points to this. Regarding its difficulty, things get better once you also download Sonic 3, because a save system is introduced. The problem is, that downloading both this and Sonic 3 in order to play a proper game requires 1600 points, or 16 bucks. Are you really willing to pay 16 bucks for a 1994's game which doesn't even come up to Sonic 1 and 2's level (both also 800 points)? If you know where to look, with 16 bucks you would be able to get a SEGA Master System, all of Sonic's Trilogy and this game along with their Game Gear twin brothers; heck, you could get everything above with 2 bucks if you bought any of Sonic Games Collection ports.
Bottom line? This game will only give you frustration and anger towards SEGA for taking a successful formula (Sonic 1 & 2) and overfeeding you to death with it for more cash. It only gets a 1.5 instead of a 1.0 because the music is catchy in a couple of Zones.