Summary of the three main areas of the game:
Campaign: Mediocre. Good level design and some nice moments, but cliche dialogue and generic storyline. The cinematic cut scenes were a big let down. That said, RTS games rarely have very good campaigns, so relatively it's really good.
Multiplayer: Good. More about speed than strategy though and other modern RTS games have a lot over Starcraft 2 here, without the vice versa. A lot of RTS games that have more strategy and more variation (Supreme Commander, Dawn of War, Universe at War etc.) That said, it's still good and well balanced. A nice league system.
Custom Games: This area made Warcraft III an amazing game. The sheer variation and level of content, a lot of which vastly outdid the core game play. There's something in here for everyone, from remakes of other games (video, board, card, mind...), to overhauls of the core game play, to the immense amount of original content (from mini-games to full fledged RPG campaigns). Since Starcraft 2 is both more popular and in possession of a better map editor, I'm sure it will do even better. Right now there's not much of this content, but given time, it will grow. Owning Starcraft 2 will begin to feel like owning hundreds of games.
The main problem with custom games was that, if you got really good at a particular type of game, it was rare to come across someone who was as good or better. I've heard it said that there will be ways around this for the most prominent custom game types, but even if those did happen, it still leaves most of the games in the dark.