If you've never played Portal before(I was one of them), then I believe it's well worth the $15. However, during the main part of the game I kept thinking that Braid was harder, had better puzzles and a more intriguing story. I haven't attempted the extra challenges yet, but will probably tonight or this weekend. One thing I thought was wierd: I pretty much played the complete original story and did not get one achievement. I finally got one, but that's it. Really tough.
BTW, I finished the main game(not the extra challenges) in one evening. The Time Challenges(Challenge Mode) will probably keep you more busy, but other than that I wouldn't expect much more replay value as their isn't any multiplayer.
Tommy McClain