"Portal" was a magnificent game - new, different, just challenging enough, and dominated by the syrupy-sweet psychosis of everyone's favourite AI. [Not to mention the best closing theme music of all time.] I was unsure how interesting "Portal 2" could be, given the end of "Portal". Would anything match GlaDos? Would it be too different? Not different enough?
Thankfully, "Portal 2" is well worth playing. There is inevitably a little bit of "more of the same", given it is based around the same puzzles, but there's just enough new mechanics to make it fresh - and fortunately I've been able to work out solutions for all so far, which helps the satisfaction level considerably. And the plot, the new characters, the tying together of old and new - all excellent.
I'm making a note here: huge success.