Bought it yesterday and played around with it a bit. Here's my first impression.
Is it very different from the original Starcraft?
It's not a modern day remake like Civ Col or Monkey Island SE but it's not an entirely new game either. The campaign is a sequel to SC1, the gameplay is largely identical with some old and quite some new units.
So what is new?
The campaign mode isn't entirely linear and apparently you have some faction alignment choices to make (not there myself). What is kinda nifty is that you earn cash with your missions that you can spend on upgrades which you would have to research in normal MP mode. So instead of researching them you just start of with them but also you can not research them in campaign mode: you need to buy them between missions. Gives the campaign a different feel from the MP game. Storyline of the campaign is really good, as far as I can see now (just started).
Also new: challenges. SP missions that are there to prepare you for the MP game. Havent tried them, but they look good and basically are extra SP content. Blizzard added achievements to the game as well, both for SP and MP. New for SC but sort of standard for all games these days.
The game ships with a heap of maps for MP, looks really good. Haven't tried online MP yet, but will for sure once I have the hang of it again.
Should I buy the game?
If you remotely like RTS, you should buy this game! The original SC was the best RTS ever imo, SC II looks like a worthy sequel. Pity it took Bliz 12 years to make it.
Oh and if you bought/buy it, lemme now so you can kick my ass.