First and foremost this game is addictive. It's taps into the human psyche to never give up and win. That would be great if there weren't some flaws with the game.
The game is basically a endless point to point scroller. A lot of games are like this and in there own right challenging. The problem with Flappy Bird is the bird is heavy. A tap barely does anything and the bird doesn't glide. In other games whether it's a bird, a spaceship or a helicopter. They all glide to a certain extent. Flappy Bird doesn't at all making the game selfishly difficult.
Then to throw salt on the wounds of us social networking addicts who decided to play the game because of our friends the hitbox of the bird is to big. To experienced fighting game gamers they know what this is. Basically there's invisible points around the bird so that the game knows when the bird collides with the pipes or ground. It needs to be minimized on the tail end of the bird. You'll feel you cleared a pipe only to end up getting a game over scream because the tip back will hit the pipe.
Also the game is not optimized for the iPhone 4. I don't know if it's apple fault or the developers fault but there's a lot of stuttering going on especially when an advertisement appears. Speaking of advertisement some of them block the pipes and it's hard to match up the distance to get through a pipe when I can't see the pipe. The stuttering is highly detrimental during crucial pipe taps.
All in all if your expecting a causal game then this is not it. If you want to be tortured brutally then you definitely have a game that you will love. It's the iPhone equivalent of Dark Souls. Definite optimization need to be done to address the stuttering of the game.