Okay. Portal is really, really good. It's a bit slow at the start (because they have to get the learning curve right for everyone - it's a totally different game) but it's a fun little puzzler, and the ending is lovely. It finishes quickly, but after that there's the challenges, and believe me, those are *hard*.
Slightly spoilerish, but those who finished the game will love this: <!-- m --><a class="postlink" href="http://www.computerandvideogames.com/article.php?id=173586&site=pcg">http://www.computerandvideogames.com/ar ... 6&site=pcg</a><!-- m -->