I think Spa Day is very underrated for a game pack compared to its successors. If you're not willing to spend your life at spas, then you'll find that you'll barely visit them like I do, but I still do enjoy the spa aspect and for builders who want to build functioning spas, and that there's even a spa venue to even place down at all. Plus, a new death type was added with the saunas, and people constantly complain about more ways to kill off their Sims so there's that.
I love the CAS and BB items that came with it, it adds a sort of sleek fit kind of feel combined with postmodern furnishings. I'm guilty of not using some of the BB items enough though, bc it is a very specific style that doesn't work in every house. The CAS items can work well with almost anyone who wears them.
The Wellness skill is actually particularly useful the higher you achieve in the skill. Not only do you sometimes will randomly increase a certain skill, or repair broken relationships, etc while meditating, you can even begin to teleport to whichever spot at will with it (and not the cheaty way of teleporting either). Also going back on the spa objects and features, you'd be surprised how long some of those moodlets can last if you get the correct treatments and such for your aches and pains, up to 8 hrs if you want to be fully in the right mood for the whole workday.
All in all, it's all in how you would want to play your Sims with this game pack. I find it very useful for what I need it for most of the time, but also understand why it's underrated compared to the other game packs succeeding it.