Upgrading to Premier is dependent on your priorities. I always upgrade, but that's because I have a FABULOUS time with the photographers. We take goofy pictures, fun pictures, formal pictures and they'll also do candids. The photos also include everyone in your cabin, so the 3rd guest will be included as well. The photos themselves are worth the cost for me, but I also enjoy the Specialty Dining, though the MDR food is wonderful.
Cool part of Premier SD is that the 4 included can be shared with cabin mates. For example, I am going on a cruise next month with my Mom and my Best Friend. Mom and I have the Premier, BF didn't get a package at all. I booked 2 SD meals, the Steak House first because it costs more. So, that reservation took the first 3 of our included meals, then I booked Sabatinis, the first meal was covered and I had to pay the 2 $35 fees for the second 2 meals. Doesn't save a huge amount, but money is money.
Premier also has the Princess Prizes. Last January we all 3(even though only 2 of us had Premier) got 3 $25 OBC prizes, just for opening the cabin door. It's possible that it was a glitch that has been fixed by now, but it was cool when it happened.
I've yet to try the fitness classes, my version of fitness on a cruise is "fitness dessert in my mouth'.
Premium Desserts were a hit with my children, they were 8 and 12 on their cruise, but 1 a day is more than enough for me. Though I haven't tried some of the new ones they've come out with. My last cruise was March 2023.