I found them all to be kinda frustrating.
Sylvan Glade (Base Game, Willow Creek): Okay, it's gorgeous, has a great spot for fishing and capturing frogs, and it's just simply cool as it's typically the first hidden world most of us find. What frustrated me was I can't live there and I was hoping it was like an easter egg telling us we'll be getting fairies soon and can build our own lots like this. Nope.
Forgotten Grotto (Base Game, Oasis Springs): Once again it's beautiful, has a great fishing spot (only place you can score the elusive Batfish), good mining for crystals/metals, a few frogs, but that's really about it.
Sixam (Get to Work, rocketship or wormhole): It's a great place to meet aliens, if you're into that sort of thing, and complete certain collections. Once you've completed the collections, which doesn't take long, unless you for whatever reason need to meet aliens there's really no reason to return. Would be nice if you could build something there, like a business.
Deep Woods (Outdoor Retreat, Granite Falls): Some rare plants, rare fishing, only place you can score certain bugs to complete that collection, and the hermit will teach you something new once a friend (also unlock a secret Trophy/Achievement if you invite him into your home), but once you've exhausted all that it's over. I love the hermit's house, and was disappointed to discover you can't live there permanently. but you can stay there for the duration of your vacation if you want (and don't mind sharing with a strange hermit). It's got everything one needs for staying a prolonged period of time; bed, refrigerator, stove, campfire, but it's not really outfitted for multiple sims (only one single bed). If you have access to this, then you probably own tents, so that can solve that issue. Because of the house that allows anyone full access, I think this is the best secret area of the ones I've discovered.
Here's my biggest frustration with these areas; even after you unlock them you still have to travel to their area to access them. What I mean is, for example the Sylvan Glade, after I unlock the area by interacting with the tree and then succeeding in the tree's events and discovering the grove, if I ever want to return I still first have to go to Willow Creek and then select the tree to get to Sylvan Glade. I wish it opened up Sylvan Glade on the map to bypass the extra loading time.
