It's a tool for the toolbox. Not as good as facing live pitching, but certainly better than nothing other than front toss.
My son liked the Pro Batter because it gave him a chance to see good quality off speed and breaking stuff. It's hard to get a ton of reps at that in normal BP.
For fun he liked to have them crank the machine up to full velocity sliders. (Fastballs went up to 100, sliders in the low 90's if I recall correctly) He actually made contact once and he celebrated like it was a world series winning walk off.
As a side note, they work well for breaking in your new catcher's mitt. Thirty minutes at 90 and that baby's almost game ready.