I suspect Starcraft II will go on to sell tens of millions of copies. It's incredibly polished and features some of the most dedicated online support ever for a video game. It lacks in quite a few areas though, such as truly new ideas or in-game community support, but ultimately it'll replace Starcraft as the dominant Rush RTS Game and as the poster child for eSports.
Ultimately, it was designed to replace Starcraft rather than sit beside it, so the fact that it's designed to place the complete control of it's multiplayer in Activision Blizzard's hands makes for a rather uncertain future. Instead of unofficial ladders, tournaments and competitions, everything needs to be Blizzard-approved. The complete lack of offline multiplayer functionality simply serves as a reminder that things have changed, and not for the better. LAN was what made Starcraft the sensation it is today, and Starcraft still features amongst the games at all major LAN events. The removal of that and hard-coded regional restrictions really destroys the community that made Starcraft what it is. Coupled with the pricing structure, uncertain community map handling features (charging for the good player made maps?) and the Activision Logo on the box, it's hard to say if Starcraft II will as great as the fanboys will have you believe. It'll be critcially hailed, commercially successful, however in twelve months time, when the next expansion rolls out, we'll see how things are looking.
I won't be buying the base game until the price drops, and judging from most Activision titles, that's twelve months.