StarCraft II is very clearly a successor to the original rather than a divergence from the game's lineage. It builds on the strengths of the original without straying into dangerous innovation. This will be welcomed or derided depending on your hopes.
The longevity and competitive success of the game remains to be seen, and will in the author's opinion depend on whether the more subtle gameplay changes breathes significant life into the game. Those who have fallen in love with the squad-based architecture of some modern RTS games may find the ability to select 255 zerglings a poor substitute.
The game plays well and is as expected extremely well polished. It is almost assuredly a worthy successor to the original classic, and I'll be parting with my hard-earned without hesitation.