Let me start off by saying I never played Portal, not sure why, it was a game that never appealed.
I traded Black Ops in against this as I was more intrigued than anything and just wanted to try something over the bank holiday.
Absolutely brilliant, an excellent game howver I am struggling to describe what game it is like.
My advice, put the kids to bed, wait until the othe half is out (or watching a long TV program), open a bottle of red and then just sit back and get your head ready.
Really easy to get into but the puzzles soon ramp up (turrets do spoil it a little for me but I admit they do add another element).
If you want flat out action look elsewhere, definately a thinking mans / ladies game.