Just rolled credits, although I cheated a bit.
To start with, most of the levels in this pack absolutely rule, if it was just a level pack and a (suggested but optional) custom Junio Sonic, this would be a 5/5 easily, only held back from "I wish I could give this a six" by performance issues and some pretty obnoxious enemy spam in some levels. (Which probably also doesn't help with performance.) The theming and spectacle is fun, the flow is frequently immaculate, and a lot of stages feel like they do a good job taking advantage of Junio's unique moveset and water running. It's honestly hard to pick out highlights, because when I attempted to do so I basically listed every level except Stormspire Heights, and even that is less "not a highlight" and more "I suspect this would be awesome if there were more checkpoints, less enemies, and my framerate didn't randomly decide to die in this level." Even the hub world, which I am about to complain about, has fun movement between levels and cool platforming challenges for level entrances and emerald and emblem locations.
This variation of Junio Sonic is also great, between the (I think buffed?) rebound dash, the drop dash, and the dash mode, along with the heavy use of slopes in the level design, I could swear there was a momentum mod if it wasn't for the lack of the hyperspeed glitch when loading one in and vanilla characters feeling like they could use one when loaded into stages. (Note: This is not a request to pack in a momentum mod. If someone wants to use a momentum mod they can pick their own, and a lot of people do not and that's fine too. Especially when the maps are designed for a specific character without one.)
Unfortunately, and the main thing holding it back from a 5/5 for me, it's not just stages and a character. I suppose the idea of a hub world and emblem collecting to open up stages at all is a matter of opinion, but even outside of that not being my preference, it's not executed very well. The hub world itself doesn't do much of anything to telegraph level locations unless you just opened up a new section, or even to communicate when you HAVE levels you haven't been to. I actually missed an entire level until cheating Knuckles in to search for emeralds, and didn't even know it was missing! On top of this, the amount of emblems required for later areas is a bit much, I actually wound up editing the total required for the final stage down because I'm pretty sure it's 100% completion, and I'm glad I did because that stage is incredible. And to make the whole emblem collection thing worse, there are no emblem hints or emblem radar. (That's another edit I made to my copy, and I'm pretty sure I just wouldn't have played the latter half of the game without it.)
I'm honestly not a fan of this entire structure, but the implementation in particular, along with performance issues and occasional enemy spam, really make this whole mod feel like less than the sum of it's parts. But also, they are VERY good parts.
Tone down the emblem collection some, tone down the amount of just the worst enemies in Vulcanic Rave and Stormspire Heights, and maybe figure out a way to improve performance (even the SD version gets huge frame drops in some stages, especially if you replay them a few times for some reason?) and this becomes an easy 5/5. Release a side version where the character lock is just a suggestion and the hub and the stages are just arranged in order as a normal campaign and this becomes an all time great.