VERY MUCH worth it. I bouthg my now 21yo son one when he was 14. I don't think he has had a "sore arm" since after pitching. Using it for 15 minutes prior to warming up before a start also cut way down on how long and how many pitches he needed to get "hot." In addition, at one point, we started using it in between innings and he swears by that use as well.
As a high school pitching coach I bought a few cheaper options for use by the team. They also helped. However, they weren't as durable. The MarcPro I bought in 2014 is still being used on a regular basis while the cheaper versions rarely lasted the entire season. In addition, the cheaper versions work by a 9v battery rather than a rechargable one and they run through batteries at a rate that probably had me spending the savings in batteries.
To this day, I still contend that the best investment I ever made in my son as a baseball player was the MrcPro.