This game is a throwback to an ancient Sonic clone on the Amiga. Because this remake attempts to remain faithful to that, various mechanics will feel a lot like Sonic 1 on the Master System, and this is why.
The default camera behaviour is terrible. This is an unusual point to mention at the start of a review, but it's the point that stands out the most as some players report it making them nauseous. Fortunately it can be fixed, although this requires tweaking a configuration file.
While you're there you might also want to disable level introductions as they waste a lot of time and offer nothing.
So now you're in the game. You'll soon find that, like Sonic 1, acceleration and deceleration are almost instant. I personally prefer this as turning doesn't take an age (you hearing this New Super Mario Bros?), but it makes some of the areas that demand precise control a bit tricky.
Early on, you may find enemies hard to see. This is really only a problem in the first chapter as the blue ones blend with the background and look like a powerup. It's not a problem after this as you get used to it and later enemies stand out more.
For the most part enemies are a mere nuisance. You get 5 hits before dying and potions that restore those completely are very common - sometimes occurring more frequently than every 5 threats! Spikes are the stark exception, as they kill you instantly and some are placed in a way that makes avoiding them awkward.
The overall difficulty is... strange, to say the least. The original required getting 80% of the coins in a level before the exit was available; that has been removed, although there is still an achievement for the feat. You get star ratings each level, and to get the maximum 3 stars you can't die. Considering some levels can take 8-10 minutes, this makes those spikes needlessly punishing.
Then again, star ratings don't actually do anything. They aren't required for progress and don't even contribute to any achievements. So unless you want to thoroughly beat the game maybe those spikes aren't that punishing after all.
Secret areas were a large part of the original's charm, and these were remembered. Levels generally have around 10 hidden in walls, which gives a nice focus to exploration. One bizarre occurrence is that when a secret area is revealed, your vertical inertia is killed. This feels untested as it means when you reveal a secret during an ascent, you'll fall before being able to enter it.
The downside to secrets is that sometimes collectables are placed behind walls over spikes. When this happens the wall is never a secret, but unless you're aware of that fact in advance you may feel like testing every time, and dying every time.
The worlds are colourful and varied although somewhat stereotypical. Some environments like the circus and the pyramid are as boring as they always are, but others like the green hills are pleasant and enjoyable. I must note that this game has one of the few ice worlds that I've actually liked in any platform game ever.
The game has a main mode with 24 levels, a further 24 levels from the original Amiga release which you'll unlock as the game progresses, and a challenge mode called Frog Trials. The main mode is the important one, the others mostly feel like extras. Obscurely every achievement is linked to the main mode too, so hunters can completely ignore the remaining content which seems like a strange choice.
Overall the game is a fun throwback for 5-10 hours. I wouldn't recommend buying it for anything less than a 75% discount on Steam, but it does appear in bundles from time to time and if you happen to have it in your library I'd definitely recommend giving it a chance.