I am writing this review primarily for those who have also reviewed it and have claimed that the controls are somehow bad.
The simple fact is that if you are finding the controls difficult, you are trying to play the game wrong. Trying to play this game like a normal platformer, staying on the ground, trying to time your jumps, and ignoring many of Gish's abilties will result in abysmal failure. Instead, your main focus needs to be on your momentum. Knowing the levels is key, so that you can plan out a route before hand which is fluid and smooth. Even a tiny change in your path can drastically change how you move across the terrain and can be the difference between leaping across a huge gap or flopping and losing all your speed. Jumping is everything. Setting yourself up properly for a good jump, even if that requires you to go out of your way or backtrack, can often lead to significantly improved times due to the extra speed you will carry out of it.
That being said, this isn't easy. Even beyond the ~15 hours I have clocked on Steam I had played the demo extensively many years ago. One of the great things you can do when you do get good at it is speedrunning it for fun, as I stated before the gameplay revolves around keeping your speed, and there are also a number of shortcuts, alternate routes, or even somewhat abusable physics bugs which you can use to your advantage.
As for other things, the Edmund McMillen style is nice, though the graphics and interface are clearly dated. There are a number of minor bugs in the physics which can be frustrating until you learn to work around them. Overall I think the uniquness of the gameplay outweighs the flaws.
I'll end with a couple tips for those who want to become better:
-tapping sticky while climbing allows you to climb much faster, it also helps while moving across the ground
-being at high speed and then pressing sticky will cause you to compress down, allowing you to get a good jump in
-holding slippery+heavy while in the tiny tubes will make you go faster through them, and sometimes you can launch out of the exit
-slippery+heavy is optimal for pushing blocks around without getting caught up on them
-while climbing, you can swing around 90 degree corners by tap-releasing sticky and using the arrow keys to shift your weight