For me it's worth it. The yoga classes are better explained and better presented than the gym yoga classes I have tried in the past with live trainers but large classes. The HIIT classes have noticeably improved my cardio fitness over the last few months (my VO2max has gone from 34.5 in December to 44.3 today, largely due to these classes). The strength classes are not designed to build muscle, but to safely build functional strength, and my strength has noticeably improved these last couple of months. It's much cheaper, more approachable and more convenient than gym membership, and much less time-consuming than trying to find and evaluate the safety, effectiveness and enjoyability of free YouTube fitness classes, because the Fitness+ classes are consistently of high quality. If I were on a low income, I'd weigh up the cost/benefit differently, but as things stand, it's worth it for me.
One of the reasons I don't do well with live gym classes is that if I have to miss a class due to illness, travel or other commitments, I feel embarrassed and awkward about it and can't always bring myself to go back to the class the next time. So I miss one class and the consequence is that I end up missing the rest of the course (yes, I suffer from social anxiety -- that is not uncommon though I acknowledge it as a weakness). With the Apple Fitness+ classes, there is no social consequence so missing a day doesn't interrupt my commitment to fitness and I can keep going. Why would you want there to be a consequence?