Had a little whirl last night after pre-loading and am absolutely loving it. It really is a nostalgic celebration, and the mixture between old and new is (so far) amazing.
Green Hill Boss killed me quite a few times until I realised what I was doing wrong.
Chemical Plant Boss surprised me as well. Did it first time though as I'm very familiar with those.
I suck at Special Stages.
Jumping on again in a sec. Really enjoying it, and I've not played a Sonic game in years.