I've used spelt flour at 40% of the bill.
As others have mentioned it imparts a nutty flavour, and a redish hue.
My recipe has long since disappeared so I can't give any more details about my batch.
I loved it, and others who sampled it also rated very highly. Once my new rig is up and running I will be using spelt again. It's tasty and unique IMHO and well worth trying.
I found my supply via a shop specialising in home baked bread. The shop in question has long since closed, but I'm sure there are others floating around.
HTH