This product is made to do the job and it doesn't matter if it's used for high heels, flats, or tennis shoes - it's well made and sturdy.  I have a bunion and a hammerhead on my right foot and after a few hours of wear most shoes become uncomfortable.  With this device I've been able to stretch all of my right foot shoes using the pegs in different holes to get the stretch just where I need it.
The longest I've left the stretcher in place is 24 hours, which probably means I bought the wrong size shoe to begin with!