If you love the original Portal gameplay then by all means get Still Alive for additional levels.

User Rating: 9 | Portal: Still Alive X360
I absolutely loved Portal when I started up on The Orange Box and once I finished the game I wanted a lot more of where that came from. So when Still Alive was announced it wasn't a tough choice to buy it from XBL. It was a welcome home feeling to finish off the original missions again. The challenge maps gave me the extra levels I always yearned for and once you're done they unlock advanced levels from the original story missions. Sure you'll miss the monologue provided by the AI GLaDOS in the challenge map section but if your reason for enjoying Portal was pure gameplay then don't miss out on this one. Minor irritants are there like load times and when you die it doesn't load the last checkpoint right away (may need a button press to initiate it).

...........THE CAKE IS A LIE............