As we live reasonably close, we visited Northumberlandia for a bit of fresh air. The sculpture is fantastic and I thought the paths being graded (easier, medium, hard) was a really good idea.
I think there's still some work to be done (no signposts, no idea what my £2 donation was paying for, no toilets, cafe, picnic benches or play area) and it will look better once some wildlife appears and the plants around grow back. I'd probably give it 5 stars after that.
They seem to be having some trouble with the walls and paths - but I expect that is due to the shocking weather we've been having lately - and I'm sure it'll soon be sorted out.
Overall, the sculpture is fantastic, the views are impressive, and it's really good to see something being made of an area formerly damaged by mining. Well worth a visit.