Portal 2 is awesome and it has one of the most fantastic gameplay experiences with depth, replayability suffers once you complete both campaigns since there is nothing much you can do except to wait for possible DLCs which are not confirmed for now. 
Newcomers to the game are encouraged to play the classic Portal. But even if you decide to start from Portal 2, you will not suffer from gameplay frights as the tutorials and learning curve is gradual. 
While the game is a multi-platform title, there are significant differences. The Xbox 360 version also does not feature cross-platform play while the PS3 version comes with a free Steam copy of the game for PC and Mac, capable of cross-platform gameplay with each other.
Portal 2 is available for retail now and will cost you S$69.90 for Xbox 360 and PS3 versions while the PC version (Steam) will cost US$49.90.
Oh, and remember, what goes in fast, comes out fast. The cake may be a lie but our review scores aren't. 
Score Rating  
Story 9.7
Visuals 7.7
Audio 9.5
Gameplay 9.7
Replayability 8.0