I have but more passing through than poking. Pretty much high desert. There is some pretty cool areas along the John Day going up to Spray, Dale, and Monument. Not a lot of people since you have to go there on purpose.
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I've ridden my adventure bike in many of those areas. From Vale to the Harney basin is pretty desolate. Turning north towards the Crooked River there is some interesting forest. Late spring would be a good time to explore it. Earlier there would be lots of mud and maybe snow. Later it is hot dry and dusty.
I have been around there. Mostly on my motorcycle. I did the OBDR (Oregon Backcountry Discovery Route) which goes through that area. There is rumored a plane wreck near the Nevada border which we tried to locate. It is on a hillside and it was an old army plane. Never found it. It is basically high desert and nothing around for miles..... and it just keeps going and going and going........ Had to plan gas stops well and have GPS!