The major sticking point for modern System Shock players is the control scheme, which uses keys instead of the mouse to look around. System Shock also indirectly sets up the story for System Shock 2 - it's the origin story of megalomaniac artificial intelligence SHODAN, whose reappearance in System Shock 2 would be a huge spoiler if it weren't right there in the box art. There's less sneaking around and figuring out ways to use enemies' strengths against them, but more wire-frame cyberspace sequences and some delightfully weird features like rocket boots.
Beyond the obvious graphical differences, it's more of a straight first-person shooter, with some cybernetic upgrades available along the way. System Shock's gameplay is not, in fact, very much like that of System Shock 2.
It's currently on sale for $7.99, or $5.99 if you already own System Shock 2. GOG has apparently tracked down the rights to the game, however, because the System Shock 'Enhanced Edition' is now available for $9.99. It finally made it onto GOG and Steam in 2013, but one major piece of the puzzle was missing: the original, lesser-known System Shock, released in 1994. For several years, the survival horror classic System Shock 2 was one of the most requested titles on vintage game store GOG.
