it's usually open at easter and over christmas/new year a few days, and there's usually a church service. The village itself is not that atmospheric really, but there's a bit of a tingle watching the people go into the church. Most of the buildings are pretty derelict, or patched up with breeze blocks, but there's an old manor house in a walled garden that appears to untouched and unused and thus mysterious in a Famous Five sort of way.
What I like is the emptiness of the plain, apart from a few dead tanks there's little evidence of the military or of people.
fwiw I did quite a bit of Cruisewatching too, but never made it anywhere near Imber, we were always creeping around on the other side of the plain, by the Bustard.