Its hard to describe this game without overusing the word "beautiful"; Braid is in my top 10 favourite games of all times and its definitely my favourite indie game.
Everything in the game seem to be an artwork: the music, the background, the puzzles, the storyline, sound effects, mechanics... its involving, gets you deep in the story if you try to follow it. Its challenging enough to get you thinking, but once you think, you get it. The puzzles are not long, everything you need to know is *there* (well, except for one single specific puzzle but that ain't a dealbreaker I assure you) and rarely rely on dexterity (unless you're interested in speed runs).
The story is fantastic and got me thinking a while, but there is a "k00l secret ending l33t for science" which I didn't find cool at all. Basically, the first immediate one is a love story and the second one... its more about work and science (no spoilers shall be given), really, and it ain't all that beautiful. That's my only complaint about the game: the regular ending leaves some things in the air that only make sense in the pr0-tröö-g4m3r secret one, but the regular ending sounded way more appealing to me, and I wish the developer would have exploited more the vanilla one.
If you like achievement hunting there isn't much challenge in this one here, except for the speed run achievement, which might need some practice but its not overfrustrating.