It's unique puzzles, incredible writing, and superb gameplay makes this a worthwhile purchase.
Portal's graphics are very good, for what I've been seeing on Xbox Live Arcade. The Portal Effects look really cool and everything has nice polish that makes it stand out a bit more. There are better looking games on the Xbox 360, but it still looks very good, for all that it's aiming for.
Portal's story takes place in the Apeture Science Research Laboratory, where you wake up, probably wondering why you're even there, when a computer named GLaDOS tells you that you must navigate an obstacle course using your wits, physical abilty, and a gun that shoots portals.
The game is incredibly well-written. Your character is a mute, but the Super-Computer GLaDOS, that talks to you with hilarious lines on your way to escape from the Apeture Science Research Laboratory. When taking out turrets, they usually say funny lines like, "I don't blame you..." Or, "Please proceed into android Hell. Yes, that is a real place."
The only real "problem" with Portal is that it's really short, but that's okay, because that means that it doesn't stay any longer than it needs to, and it has infinite replay value just to hear some of the best humor in video game history.
Final Score: 9.4/10
Good and Bad
+Clever Portal-based Puzzles
+Strong Central Plot
+Laugh out Loud Funny
+Steady Difficulty Curve
+Somewhat impressive graphics
+Good length for a 10 dollar game
+It's Not Very Expensive
+Cool Sound Effects
+Still Alive End Credits Song
+Wraps everything up very well
-THE CAKE IS A LIE