I finally got around to playing the Portal series. I played 2 first (it was a free Games with Gold pick) and thought it was great, so about 20 or so puzzles through the first Portal now. I'm kind of surprised the series ended with one sequel.
There would probably be 4 sequels by now if it was by any other company but Valve. Sadly, their priority has been Steam for too many years now. Their game development side goes at a glacial pace and all the developers that originally made their critically acclaimed games have left or are leaving.