My usual response is that it really depends on your playstyle.
I think that as a pack, Spa Day is kind of niche. It's not really something to generally appeals to people. In my case, I like it well enough, but I only send my sims to the spa every once in a while. The CAS items aren't so bad either. While it's a nice thing to do on your sims' day off, the spa objects just give happy moodlets and don't really affect the core gameplay that much. Yoga and meditation are more accessible at home, although AFAIK, their effects are limited as well. I think if your wellness skill is high enough, your sim can teleport while meditating like in Sims 2, but I haven't tried this out myself.
My advice is to look over the CAS items and gameplay videos. Think how much use you'll get out of the pack, and decide if it's worth 15 bucks to you. :)