There's a reason why this franchise is still around and it all starts here.
The gameplay is fairly simple. You run and jump around levels, trying to collect as many gold rings as possible in the shortest time, all the while avoiding the numerous enemies and obstacle and eventually facing the boss, Dr. Robotnik (or Eggman) in different fashions.
What makes Sonic really interesting is the number of paths you can take in order to complete a level. Some are shorter while others have more gold rings scattered in them but no path is really better. This adds great replay value. Also, there are number of bonus levels between the standard ones where you try to collect rings and special chaos emeralds. Get all six emeralds and...well you'll see.
The graphics are simple, yet very colorful. The music is an awesome part of the game, considering the time at which the game was made. In fact, you'll probably have some tunes stuck in your head for a few hours.
The only downside of the game is the lack of any saving feature. You close the game, you start again. Sure, there's always cheats to access levels but it's never the same feeling of accomplishment than if you had done it all yourself.
In overall, the original Sonic the Hedgehog is a must for any nostalgic gamer. It is, after all, the genesis (I know its a cheap pun) of a great series.