I like Spa Day a lot. It is one of my favorite GPs.
I added two of the spas that came with the pack into my game. One is a general spa with various things that you can do that I put into Newcrest; the other is a yoga ashram that I added to Bedrock Straits in OS. A lot of my sims do yoga and I created a subplot of a creepy yoga cult using the GT club system. (Yeah, I know that yoga isn't normally like that; they are deviant and hilarious).
I find that a combo of yoga and jogging makes a lot of my female sims look great. I also have some of my male sims do yoga or hang out at the spa. I sometimes use the yoga routines to induce specific moods such as focused, inspired and energized. The yoga instructors tend to be attractive in my game so several of them have married into my rotation or been romantic interests of my sims. They are also good potential spouses because they aren't bugged/non-aging townies.
Max Villareal is a vampire in my game and he uses some of the daylight hours of his eternal life to do yoga and weight-lifting. Like I said, adding yoga tends to be a great thing. I also find the group yoga classes to be a bit fun to watch. There will usually be a sim with a funny expression, "Nope, I'm not trying that."
I like what this added to the game. It's a realistic type of gameplay that offers more options for my various sims to do. I also find it to be replayable, personally, but as others have said, your opinion will depend upon your play style. I like immersive realism so I liked this one a lot. (Yes, I did finally cave on getting Laundry Day, though I find that theme unexciting). I also agree with the people who said they use the baths a lot. Many of my sims will use the baths from Spa Day. My pregnant sims, for example, relax in a bath sometimes as a way to put them in a good mood. If they decide to have a baby at home, I sometimes send them into the tub while they are in labor.