So far:
Green Hill - not bad, not good. Standard Green Hill with a spin. Loved seeing the remix of Green Hill Act 3 (original) on Act 2. I tested it to see if it was the same. Did my hidden low route I liked when I was a kid, and yup. It was the same. Then I went to higher ground. Excellent art design here.
Chemical Plant - so far the star of the show. Act 2 is great.
Studiopolis - I hate casino stages and this is no different. Boring.
Flying Battery - I've always found this stage to be among the worst in Sonic 3 and Knuckles. It's plodding, drawn out, and boring. Mania does nothing to fix this defecation. Sandopolis would have been the better pick but then they had to make their own sand stage.
Press Garden - it's ok. Not a fan of the gimmick.