The sound is not for everyone
With the volume on, as you touch tiles, piano keys play somewhat randomly in time with your taps. While they mostly follow the same scale, I heard a few sour notes peppered in that made me cringe -- and often lose the game as a result. It probably won't be a problem for everyone, but if you notice the sound is distracting, I recommend turning it off in the settings for a distraction-free experience.
The best games are the ones I don't expect
As a software and games reviewer, I expect certain games to do well in the App Store. When a new version of Angry Birds or Cut the Rope is released, I can be pretty confident both will reach the top 10. Similarly, with the games that have big production values like the Modern Combat games or Blizzard's Hearthstone, you know people are going to download them.
With single-developer games like Piano Tiles, you never know what combination of gameplay or graphics is going to capture the minds of players, but it's always amazing to see these simple yet strong concepts see such enormous success. It gives hope to every aspiring developer out there that with the right concept, they too can find success in the App Store. But it also gives hope to players like me, who can be pleasantly surprised to find a game nobody has thought of before, and have it be just the type of game that makes you say "just one more round."
Piano Tiles is a game you never would guess to reach the top 10, but it shows that there really is no predicting what people will like. With the right blend of simplicity and challenge, it doesn't necessarily matter whether the graphics are top notch or if the game comes from a big company -- the only thing that matters is if you can capture the right set of circumstances that make a game simple, but endlessly addictive.
Piano Tiles is that game. It's incredibly easy to pick up and play and hard to put down once you start. If you're looking for a simple time waster, this game has just the right mix of gameplay and challenge that will keep you coming back for more.