I've read many reviews about people comparing this game with its sequel, and basing this comparison to basically not recommending it.
I'm in the point that I can comprehend both visions. Indeed, this game is unfair, the hitbox is confusing until the end, the lack of checkpoints makes it hardcore, it's more based on memory than reflexes, and as a rhythm game, it's basically a failure. Runner 2 fixes all that stuff, so it's logic that many people around here suggest you should avoid this game.
I must remind you all that Runner 2 is actually the odd member of the family. Apart from it, ALL Bit.Trip games are this mercyless, and even more (in fact, the Runner games are the easiest ones from the franchise IMO). So Bit.Trip Runner is just another game similar to all those from the same company. It's not fair at all to compare it with a game that has very few in common if you look coldly.
Personally, the Runner's games are the only ones from this franchise that I enjoyed. And if you have to choose one of them, it's just a matter of interests: if you wanna beat a real challenge and prove yourself you can face a hard game, try Runner 1; if you want to enjoy those kind of arcade games where you main purpose is avoiding things without getting frustrated, try Runner 2. I suggest playing both though.
In conclusion, you'll read many positive and negative reviews, and all of them will be true. This game is simply not made for everybody, only for people who enjoys challenges from videogames, while its sequel is suitable for everybody.