I think the pack is very nice! I'm fine with Mermaids as they are. I wasn't someone who absolutely needed them, rather they were more a very pleasant surprise. Plus, for being a feature of a pack where Mermaids are a part but not the focus, I think they're very nicely fleshed out. The island itself is absolutely stunning. The build/buy mode additions are great, and I love the new CAS items as well. That all makes the pack absolutely worth it for me. It's just enjoyable to wander the world, and to build. I like the new NPCs a lot as well.
I haven't delved too much into the conservationist career yet, but I love the patched in part time jobs and the new part time jobs and odd jobs system that come with the Island Living. It's a great setting for off-the-grid, which I love, and that shipwreck is great.
My only real disappointment is that being a lifeguard is not a playable career. But you still get to use some lifeguard interactions as you progress, and that's something.
But really, The world is beautiful. Build items and CAS are beautiful. Mermaids are a lot of fun and very well designed! Also, you can do interesting things like have a mermaid coping with living in the city and summoning rainstorms (if you have seasons) to fulfill hydration needs, and putting the shipwreck in a desert for maximum confusion/storytelling purposes.
I could use with a more potentially lethal volcano since its effect is basically an optional lot trait anyway, but it looks so good and I love it anyway.
All in all, I think it's a great pack! It's up to you to decide if it's for you are not, but I think it's lovely.