Sonic the Hedgehog has so much spirit, that even when it ever so often stumbles (or kills you), you just can't get mad at it, because you know its heart is in the right place and that it will bring back that smile on your face soon enough.
Pros: Sonic's original design is a thing of beauty, fantastic and catchy music, variety in both level design and gameplay straight to the end, multiple paths, instead of copying Mario, it did its own thing and succeeded, fast levels are great, slow levels are great...
Cons: ...the ones that mix fast action and slow platforming - those have issues, the zoomed in camera works against the blistering speed in the faster levels/sections, the slippery movement has some issues on the slower levels that require pinpoint precision, in-game story is almost non-existant, the original game had no save function, nor did it give you cheat codes for later levels, while it was full of devilish design decisions that would often spell death on your first time around