I've been down in the Depths since the start of the game, exploring the area as far as it lets me. While I can't comment on anything above ground yet, the Depths is both awesome and disappointing. Exploration is at its finest in the dark, making a path with bright blooms and looking around everywhere to spot the next light root. The repetition, copy paste everything is the disappointing part.
However after a while you realize it's basically the same map as Hyrule with all water turned into walls. A good idea on paper, but it lessens the feeling of discovery (you can even see the 'walls' on the Hyrule layer which doesn't hide water on the undiscovered map) and adds a lot of backtracking to get around a river. Plus some parts can't be reached at all without going back up first. Luckily the sheer vertical nature of the Depths still adds a sense of accomplishment to make your way around.
One big negative is the lack of persistence. There's no point marking areas with bright blooms, lighting a path, it all disappears after a while no matter where you stick the bright blooms. Everything eventually resets making everything you do feel kinda pointless. Also I keep running into all these things in the Depths that I guess only work after finding a quest up top. I'm currently exploring the Construct factory, yet keep running into locked gates, get access denied on the activation things. Still nice to find something else than copy paste Mines / Yiga camps / mine camps.
Without going up top only the colosseums work, the rest is all inactive or inaccessible. The flux constructs work as well, but I see no point in fighting them, nor anything else really. The random bokoblin / lizalfo spawns provide the best weapon attachments and spawn so frequently that's all I use. Sword from a statue fused with an arm combined with puff shrooms and combat is trivial. So I spend more time directing them to fight each other with muddle buds. Watch the chaos from the comedy AI in action.
And I agree about the rewards. It's either a piece of clothing from a prior game or something I already have 1000+ of. And in the rare instance it's a weapon, shield or bow, those are just rentals. I pretty much stopped using ultra hand because nothing lasts. It either resets straight away if you wander off just a few steps too far or it starts blinking and despawns all too soon. A balloon doesn't even last from the lowest ground level to the ceiling... Yes, it's a big sandbox, but it might as well been a very small sandbox without any form of persistence.
Anyway back to mapping the Depths, another week and I should be ready to head back to the surface!