Seeing as how this project is apparently canceled, I suppose it would be best not to think of this as a proper criticism post, as no further progress will come of this level pack anyway. Instead, just think of this as a thoughts and impressions post. Specifically, I am referring to v6 in this post, and not v5.1.
With that out of the way, here are my said thoughts and impressions:
Visually aesthetically, this level pack is absolutely beautiful. Moreso in my opinion than the vanilla campaign. These levels feel more alive and vibrant than ever before. So much so that I would comfortably say that it is the main campaign levels that are in dire need of living up to these levels visually rather than the other way around. I especially like the new emerald alters, and wouldn't at all mind something similar being implemented into the vanilla campaign someday, perhaps as a method of obtaining some secret unlockable if all of them are found and activated.
In terms of level design, I generally feel as though these levels are a step up from their older counterparts, though the tendency of levels to be more uphill than downhill is still quite prevalent and noticeable, causing Sonic to still feel rather sluggish in many of these levels compared to the more vertically gifted characters. Nonetheless, the addition of slopes is a massive plus, and the general reimagined feel to the levels is utterly fantastic.
The boss levels on the other hand, at least after the first one, feel rather lacking. SRB2 has come a long way since the old days, and there are many new bosses in the game. I would very much have preferred to see some of those get used in some creative new way rather than just the Eggmobile getting reused several times over. This was something of a disappointment.
Overall, this generally tickles my nostalgia in a more positive light than a negative one, and I even somewhat find myself sad that we will probably never get to see the other levels get likewise reimagined. I can only hope that in the future sometime, the vanilla campaign can borrow a little bit from this visually.