I have a basket of those and use them with my students and my 14-yr-old brother (he is 5'6") fairly regularly for drills, but NEVER to hit live balls/play points with.
They are definitely harder than regular balls, although they are as good as pressureless balls get. Using them for regular feeds or hand feeds are great, because the weight of the ball sort of balances out the lack of pace compared to a normal shot.
I tried hitting with them once, and the initial shock of striking against them was enough for me to stop immediately after only one rally. I then quickly switched back to pressurized balls.
They are definitely worth buying though - just make sure you are disciplined about when to use them.
2 x Tecnifibre 325 VO2 Max
WeissCannon Silverstring @ 43lbs