I just got a 5S a few days ago and the lock button is REALLY hard to push. My iPhone 4 lock button took no effort. I have to grip this 5S pretty darn hard.
The 5s lock button assembly is a slightly different design to the one used on the 5.
I find that my 5s and my wife's 5s lock buttons are quite "low" and while not flush with the antenna band they are lower than our 4s's that went before.
My friend who just got his 5 back from Apple for the lock button fix that was rolled out also said his new lock button is lower than his previous one so I wonder if the 5s assembly is being used to repair eligible 5's too.