The fidgety timing required to solve some puzzles diminishes what is otherwise and incredible platformer.
It is a puzzling platformer something along the lines of Mario vs. Donkey Kong or the Wario Land games on Nintendo's handheld but presented w/ a twist: Tim can reverse time. Many puzzles and indeed the story itself hinge on this twist. It is not a gimmick, it is a central game mechanic and it's fully incorporated on nearly every single level.
The levels themselves are each sections of six different worlds. In addition to the time reversal aspect, each world introduces an additional mechanic which must be mastered in order to solve the puzzles presented there in. All of these are unique and, again, built into the levels themselves from the ground up.
While the majority of Braid's levels can be traversed quite easily--indeed many of them can be passed through simply by holding right--the challenge comes from collecting puzzle pieces which are collected by solving various puzzles from each of the level. This puzzles are oftentimes very challenging and generally require heavy use of the game's time-reversal mechanic as well as the various level-specific mechanics.
A good half of these puzzles are very well designed. The solutions are not obvious but when they're found, the elements snap cleverly into place in a very satisfying manner. The trouble is some solutions require extremely fidgety timing and can be very frustrating w/o offering much in the way of fulfillment when they are completed. Braid's overall mood is quiet and thoughtful. Having to struggle w/ some twitchy little fractional second difference in timing to solve puzzles just doesn't work in its favor.
Aside from this though, there is very little to find fault w/ in Braid. It is gorgeous w/ great sound design. It's unique. It's challenging. It's different w/o being gimmicky and an overall very enjoyable way to spend the half-dozen or so hours it will take you to get through it. I do feel that mistakes were made in the final level design but even though I felt a good deal of what I think was needless frustration, I would not hesitate to recommend this to any gamer interested in any kind of gaming at all.