Very charming and original, even to this day. Level design shows its age in that there are lots of automated bits and the platforming is very vertical and rudimentary, but fortunately there is very little waiting. Physics don't replicate the genesis but they're perfectly serviceable as their own thing. Bosses are surprisingly good, and cutscenes, while dated, are brief and enjoyable. The soundtrack is very good.
Biggest complaints include terrible and pointless special stages, an insultingly easy super sonic final boss in which your rings do not drain (I think this was added in Plus), bad enemy design, and some gimmicks that, while great ideas, control poorly (such as the Yoshi's Island fuzzies in Star Shores Zone and the gravity reversal in Arcane Altitude Zone). I also just hate Cloudy Crowd Zone in general.