Daily user here. AwayNatureSoundstoSleep and the others in this series, so to speak, all have a fantastic design that is beautiful, interesting to play with, and have a practical, intuitive interface.
The sounds made by AwayNatureSoundstoSleep have never seemed mechanical or repetitious like some other ambient noise apps that just loop a noise sample. It does do odd volume things once in a while, but 99.9% of the time it does what you need it to.
I don't think it has ever crashed on me.
Update: knock on wood. Sometime soon after my original review AwayNatureSoundstoSleep started crashing about half of the time I lock the screen (on both iPhone and iPad), forcing me to unlock, reopen and lock again. I still think it's a great app, and it hasn't crashed except when locking so at least it doesn't disturb you once it's running right.