Is Sims 4 Spa Day worth it?

I would say it depends on your style of play. I like the Wellness skill but over all I rarely use most of the pack. To be honest I tend to forget to place Spas in my worlds and if I have them I rarely have sims that it makes sense to send to them anyway. I would mostly recommend it if you like modern style build/buy mostly because it does bring a lot of that to the table. However if the only thing that appeals to you is the hair then it's probably not going to be worth the money for you in the long run and you're better off searching for CC along the same lines.