Well, depending on what video card you have, you get varied results.
I have a GeForce 6200, and most of the levels are playable.
Hero
Action Stage - City Escape
Playable? Yes
Difficulty? Easy-Medium
Notes? A few glitches at the start and rarely around the level, it's easily finishable, but it's harder to do tricks on the ramps.
Boss Stage - F-6t Big Foot
Playable? Yes
Difficulty? Easy
Notes? There are glitches around, but it's very easy to ignore them and defeat the boss normally.
Action Stage - Wild Canyon
Playable? Yes
Difficulty? Easy
Notes? A few glitches but otherwise playable.
Boss Stage - Doctor Eggman
Playable? Yes
Difficulty? Easy
Notes? Completely playable, no glitches at all.
Action Stage - Prison Lane
Playable? Yes
Difficulty? Medium
Notes? More glitches than some other levels, and you may get lost near the end.
Action Stage - Metal Harbor
Playable? Yes
Difficulty? Easy
Notes? Few glitches at all, the ones that do appear are very small and won't bother you.
Boss Stage - Shadow the Hedgehog
Playable? Yes
Difficulty? Easy-Medium
Notes? A few glitches that might bother you a bit, but with practice you'll live with it.
Action Stage - Green Forest
Playable? Yes
Difficulty? Easy-Medium
Notes? A bit of glitches, there's one particular one in the springing area that will bother you, but gets easy with practice.
Action Stage - Pumpkin Hill
Playable? Yes
Difficulty? Easy
Notes? A little bit of glitches.
Action Stage - Mission Street
Playable? Yes
Difficulty? Easy
Notes? A little bit of glitches.
Action Stage - Aquatic Mine
Playable? Yes
Difficulty? Easy
Notes? A little bit of glitches.
Action Stage - Route 101
Playable? Yes
Difficulty? Easy
Notes? Barely noticable glitches.
Action Stage - Hidden Base
Playable? Yes
Difficulty? Easy-Medium
Notes? A bit more glitches which can obstruct viewing at times.
Action Stage - Death Chamber
Playable? Yes
Difficulty? Easy
Notes? A little bit of glitches.
Boss Stage - King Boom Boo
Playable? Sort of
Difficulty? Hard
Notes? You can't see King Boom Boo's shadow, making it hard to hit him.
Action Stage - Pyramid Cave
Playable? Yes
Difficulty? Easy
Notes? A bit of glitches, but easy to ignore.
Boss Stage - Egg Golem
Playable? Yes
Difficulty? Easy
Notes? No glitches at all! Completely playable.
Action Stage - Eternal Engine
Playable? Yes
Difficulty? Easy
Notes? A bit of glitches.
Action Stage - Crazy Gadget
Playable? Yes
Difficulty? Easy-Medium
Notes? Glitches can obstruct view.
Action Stage - Meteor Herd
Playable? Yes
Difficulty? Easy
Notes? Hardly any glitches.
Action Stage - Final Rush
Playable? Yes
Difficulty? Easy-Medium
Notes? Glitches can obstruct view a little bit sometimes.
Boss Stage - Shadow the Hedgehog
Playable? Yes
Difficulty? Easy
Notes? Hardly any glitches at all, if any.
Dark
Action Stage - Iron Gate
Playable? Yes
Difficulty? Easy
Notes? A bit of glitches.
Boss Stage - B-3x Hotshot
Playable? Yes
Difficulty? Easy-Medium
Notes? Glitches can obstruct view.
Action Stage - Dry Lagoon
Playable? Yes
Difficulty? Easy
Notes? Hardly any glitches.
Action Stage - Sand Ocean
Playable? Yes
Difficulty? Easy
Notes? A little bit of glitches.
Action Stage - Radical Highway
Playable? Yes
Difficulty? Easy
Notes? A few glitches.
Action Stage - Egg Quarters
Playable? Yes
Difficulty? Easy
Notes? Few glitches, if any.
Action Stage - Lost Colony
Playable? Yes
Difficulty? Easy-Medium
Notes? A bit more glitches.
Action Stage - Weapons Bed
Playable? Yes
Difficulty? Easy
Notes? Hardly any glitches.
Boss Stage - Miles "Tails" Prower
Playable? Yes
Difficulty? Easy
Notes? Perfect! Completely playable.
Action Stage - Security Hall
Playable? Yes
Difficulty? Easy-Medium
Notes? Glitches obstruct view sometimes.
Action Stage - White Jungle
Playable? Sort of
Difficulty? Medium-Hard
Notes? A lot of glitches, wireframe is needed on one part.
Boss Stage - Sonic the Hedgehog
Playable? Yes
Difficulty? Easy
Notes? A little bit of glitches.
Action Stage - Route 280
Playable? Yes
Difficulty? Easy
Notes? Very little glitches.
Action Stage - Sky Rail
Playable? Yes
Difficulty? Easy
Notes? Very little glitches.
Boss Stage - Egg Golem
Playable? Yes
Difficulty? Easy-Medium
Notes? Glitches obstruct view.
Action Stage - Mad Space
Playable? Yes
Difficulty? Easy
Notes? Little to no glitches.
Action Stage - Cosmic Wall
Playable? Yes
Difficulty? Easy
Notes? Little to no glitches.
Action Stage - Final Chase
Playable? Yes
Difficulty? Easy-Medium
Notes? Glitches obstruct viewing.
Boss Stage - Sonic the Hedgehog
Playable? Yes
Difficulty? Easy
Notes? Little to no glitches.
Last
Action Stage - Cannon's Core
Playable? Yes
Difficulty? Easy
Notes? A bit of glitches.
Boss Stage - Biolizard
Playable? Yes
Difficulty? Easy
Notes? Completely playable.
Boss Stage - The Finalhazard
Playable? Yes
Difficulty? Easy
Notes? Completely playable.
Other
Action Stage - Green Hill
Playable? Yes
Difficulty? Easy
Notes? A few glitches.
That's with a 6200.