I don't play an abundance of puzzle games, so I'm no authority on the subject, but the puzzles in Braid are fundamentally different to any I've ever seen. Most puzzle games, or puzzles in games, that I've played involve manipulating objects in space in one way or another (some games use classic word or logic puzzles it's true, but these work just as well on paper and don't really benefit specifically from being video games, and again are nothing new).
In contrast, Braid's puzzles are all about manipulating time. It's a different type of thinking, I believe. Rather than considering spatial relationships, a skill that comes naturally to us as it's part of our everyday lives, you need to consider causal relationships - something that we don't have as much varied experience with since in our lives time typically only flows in one direction.
Games that involve time manipulation are not new, but the most taxing puzzle I'd previously seen in one was probably rewinding time after stepping on something that just fell down so it would carry me upwards. Meanwhile, the puzzles in Braid are complex and just unlike anything I've experienced before. And just about every one gave me that lovely little "lightbulb moment" of understanding as the pieces suddenly clicked into place in my head.
Each level has a new rule or mechanic, always involving time, that you need to understand in order to solve the puzzles. This keeps things fresh and keeps the experience escalating. Worlds often start with a couple of simple puzzles to help you understand the new mechanic. These first puzzles follow a couple of repeated patterns, the familiarity making them easier to solve and thus and excellent and natural transition into the new world. The last level stands out in being less a puzzle and more a test of speed and reflexes, which might frustrate some people, but it works very well in suddenly introducing a tension that had been missing the rest of the game, and thus giving the final level a weight and sense of importance.
I found the game quite difficult; enough so that I left it for quite a while after failing to find solutions to some of the last few puzzles. I did manage to finish it without any outside help in the end - although I still don't know how I actually solved one particularly fiendish puzzle, I pretty much muddled through that one by luck and wouldn't be able to do it a second time! So for me I would say the difficulty was close to perfect, but I cannot judge how challenging other people will find it.
So unless you're quite a bit more intelligent than me, some puzzles will take time, and a lot of trial and error, to solve. It's quite easy to just not feel like like putting the effort in, to just put down the controller or switch to something more straight-forwards, before you really start to "get" the game. Please don't; persevere and you'll most likely be glad you did.
It occurs to me that some people might not agree with the lack of a hint system (at least I don't remember seeing one). Personally I'm grateful; if I had been offered an easy way out I might have succumbed on some puzzles, but now that I've finished it I feel some level of pride that I solved (almost) every puzzle. Only the very last world is inaccessible anyway, plus there's always the internet.
Unlike some puzzle games, Braid doesn't just get by on the bare minimum of game elements; like a precious stone, every facet is finely polished and fits perfectly in the whole. The controls, while simple, are easy to use and will be familiar to anyone who has ever played a videogame, and easy enough to pick up for the rest of us. The graphics and music are some of the best I've encountered in a 2D platformer. The narrative is engaging, told less through words and more through every part of the game itself.
The entire game looks like a beautiful water colour painting. Bright colours yet soft tones and gentle, almost hypnotic movement in the backgrounds conspire with the lovely music to create a gentle, almost relaxing atmosphere that is not threatening to non-gamers and a nice change of pace for the rest of us. There's a strangely haunting quality to the game which comes across most strongly in the journal entries and the art on the jigsaw puzzles you assemble. It recalls distant memories of happier times; a nostalgic longing for the joy and simplicity of youth, a desire to recapture that perfect moment that we never fully appreciated at the time, if it ever really existed at all that is.
Which is actually a large part of what the game is about. The protagonist is searching for something; something in his future, or maybe in his past. He's not sure what it is, sometimes he thinks he knows, other times it seems less clear. In this way he's much like most of us. While this is initially established by the diary entries at the start of each world, it's supported by just about every part of the game. The old-fashioned 2D platformer mechanics, killing enemies by jumping on their heads, seeking a princess in a castle, the bright colours and and gentle musical; all these things speak to our nostalgia for the early days of video games, the same nostalgia that keeps us playing technically simple platformers in these days of big-budget 3D extravaganzas.
The animated backgrounds and constant musical accompaniment serve yet another purpose though; combined with other subtle audio and visual cues they help to reinforce the effects of manipulating time. Leaves fall slowly downwards, steam rises from distant geysers, but rewind time and suddenly those leaves are falling upwards, the steam being sucked back in to the ground; move forwards again and it's all back to normal. This feedback helps you stay oriented while you rapidly move backwards and forwards in time, as well as just generally helping to immerse you in the game.
If there's one thing I didn't enjoy about the game, it's simply that the narrative gets less coherent as the game progresses. I believe this is deliberate seeing as the laws of the game's worlds themselves get more abstract as the game progresses, altogether moving away from the comfort of the early levels to a sensation of being lost and on unfamiliar ground. While it's all very impressive and engaging, it doesn't leave you with a satisfied feeling of closure at the end. While this might be it's intended purpose, it risks alienating players. Personally I have mixed feelings about the end, but that's not necessarily a bad thing.
Overall I give it a 10/10. Every part of the game is perfectly designed to work together to create a unique and memorable puzzle game.
The last level is... something else. A complete change of pace and very unexpected, it really screws with your head. I found it quite amazing, the way the entire last world was running backwards, then suddenly it stopped and all of sudden the fact that time was going the wrong way wasn't just some meaningless gameplay mechanic, and the sudden shift in perspective re-framed not just the final sequence, but to some extent the entire game. The truth is I'm still not sure I understand what it was all about in the end.