I said it was probably outdated, so I guess
download 2.2.9 (possibly even
2.2.8) and use the exe with that.
Accel Momentum was made for an older release. I have it installed and working with 2.2.9
I'll probably make a new folder for it soon then.
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Okay so I've given it a try, and I thought I should edit this post with my thoughts. I played around in the provided test stages (3DTest and 2DTest) as well as in GFZ1 and GFZ2. I was playing in simple controls using a DS4 controller with the following setup:
Jump: X
Spin: O
Move: Left Thumb Stick and D-Pad.
Turn camera left: L1
Turn camera right: R1
Generally, the physics do actually feel really nice to me. I feel very in control of Sonic and the other characters. In particular, I noticed that using the D-Pad or Thumb Stick to move was ideal for navigating around enemies and obstacles and generally precise movements with the camera pointed in one direction, while turning the camera with L1 and R1 allowed me better general turning both on the ground and in the air. However, I did feel as though it was a little too easy to halt your momentum in midair, leading to me failing to land some badnik bounces while acclimating myself to the new physics. That one ramp in GFZ2, you know the one, felt much nicer to spin through than it does in the normal physics, though I did feel like I might have launched higher and landed closer? I'm not entirely sure.
I appreciate the ability to maintain speed after launching off a horizontal spring, though an oddity with it is that this actually seems to be faster if you remain on foot as opposed to spinning. The same thing applies to dash panels.
Another small oddity is that it seems to be just a little too easy to launch yourself off slopes. While you can more easily use this to jump at the right time and gain great height, it sometimes leads to getting launched slightly when you were actually intending on remaining grounded while running up hill. This wasn't a big problem in GFZ, but in levels with more aggressive badniks this could potentially cause the player to become vulnerable in midair unexpectedly without a means to damage enemies. A suggestion I have regarding this would be to keep players grounded if they are in their running state up a rounded slope, but launch them if they are spinning.
I should also talk about the Thok: At first, I thought the thok was god awful with these physics. It feels really slow and generally pointless to use at first. Attempting to do some badnik bounces with it proved disappointing. However, toying around with it more proved more useful than I expected. While it still wasn't the launching juggernaut that it is in the normal physics, the ability to instantly redirect my movement in any horizontal direction did prove useful, especially in 2D. I do see potential in a Thok rework as opposed to outright replacing it, though I will admit it does feel very weak compared to other characters abilities and would probably be better off dedicated to a side ability rather than Sonic's main ability.
Speaking of 2D, this also generally felt nicer to play than the normal physics. In fact, I'd say these 2D physics are far superior. However, they aren't quite perfect. It seems as though trying to reverse your direction in midair after getting launched by a dash panel (and I assume springs as well?) results in you at best being able to fall straight down. This does make the thok useful for this purpose, but it still generally feels a bit off. I feel like in the 90's classics, you'd have a hard time slowing down in midair after being launched like that, but if you could manage it you'd still be able to reverse direction. Other than that though, it felt very nice and in control. I am curious how these physics would perform on circle shaped platforms of various sizes though.