I caved in today and bought it. I didn't get to play it for too long, but it seems fun. My sim invited her boyfriend, her teen daughter, and teen daughter's best friend. They only stayed for one night, as I didn't want to waste vacation days, so they stayed on a Saturday night. I haven't read any guides for it, so it was a little confusing at first. I sent them on vacation without buying tents etc, so I had a bit of trouble finding the rangers station to buy supplies. Plus buying tents was confusing as it doesn't actually say how many sims can sleep in a tent, so I ended up buying four (luckily my sim is rich), when in fact I think the tents I bought sleep two sims.
The non-household sims that came along seemed to hang around my sims, although they did wander off a bit. My teen sim and her friend went to the national park, and when she returned back to the campground, her friend didn't come back straight away, but eventually did and slept in the tent.
I didn't do much exploring, and when I finally did make one go for a hike, she was on her way back when the vacation ended.
What annoyed me was I planned camping outfits, and every time they travelled, it would change back to what that outfit previously was. I saved in outfit 5, and they'd travel and it would be the previous outfit 5, as when I clicked to change it (thinking they went to default outfit 1), it said they were already wearing it, so I would have to change to another outfit, then back to 5 and then it would be the camping outfit again. That is a minor issue, though.
So although I haven't played it much, it seems fun and adds some new gameplay. I'm not sure yet if it's worth the $30 au I paid, although it does have a lot of content, it's mostly camping related.