8:01 am tomorrow here in Portugal.
A big problem I had with Sonic was that the track design isn't very interesting, it's cool to look at, but it's nowhere near the tightness a Mario Kart title has. Not to mention that you have several shortcuts, underwater sections, areal sections and anti-gravity, so not every lap has to be the same. With that said, Mount Wario is awesome and I wish there were more tracks like that.
I agree with you that the Sonic tracks generally weren't anything special - although there were a few really cool ones - but I feel like Nintendo could have done something truly incredible if they'd combined Sonic-esque dynamic tracks with the new anti-gravity mechanics. I'll no doubt put a ridiculous number of hours into MK8 and enjoy every one, but there's already a niggling feeling of what could have been.