Portal: Still Alive is a fantastic, mind-bending puzzle game, and an absolute joy to play.
It's hard to recommend this to anyone who has already played the Portal in The Orange Box, seeing as it costs a steep 1200 MS Points ($15). However, if you are like me and have never played Portal this is a great game, and easily one of the best available through Xbox Live Arcade. Solid graphics, extremely unique and disorienting gameplay, and some good humor make this a well rounded title. My only complaint is that it's too short, and there really isn't much replay value here. As the puzzles are only really good once, because after that you already know what to do.
This is strictly a singleplayer game, so don't except any kind of multiplayer. The gameplay basically consists of using your handheld portal gun to form orange and blue portals, these portals connect to one another and allow you and objects to pass through in either direction. The puzzles start out pretty simple but become increasingly difficult and require more thought and fast reaction time, though the main objective always remains the same; proceed to the exit at the end of each level by any means necessary, putting your problem solving skills to the test in doing so.
Anyways, this is a great game and one that NO ONE should miss, my only regret was that I spent $15 buying it through Xbox Live Arcade when I could have gotten almost the same game plus all of Half-Life 2 and Team Fortress 2 for just $15 more ($30).
So basically, while this is a fantastic game that everyone should try at least once, it's not worth buying the Xbox Live Arcade version of the game unless you are a die-hard fan of the original Portal, and just want to play the extra challenges. And if you haven't played Portal I still can't recommend this download. Seeing as you can just pay an extra $15 and get The Orange Box which also includes Portal.
I bought it on impulse having never played Portal (and having forgotten how much the price had dropped on The Orange Box). So, while Portal is a must play game, I would recommend you buy The Orange Box. The only real reason to get the Arcade version is if you are just completely against owning Half-Life 2.
Anyways, I've kind of derailed and gone on a silly rant about the price and such, but to summarize; Portal is a must play game, but you're probably better off playing it in The Orange Box than on Xbox Live Arcade. That said however, it still easily remains one of the best games on Xbox Live Arcade, and I would more readily recommend it if The Orange Box still cost $60 but it's down to around $30 retail now so you might as well just buy that instead.