I have tried one. I picked up an older IM-7? off of Ebay for $75… the guy said it worked fine, but the display was a flip of the coin. I ended up just tightening up a couple things… and she worked great.
Those things are amazing… I actually used it to learn what my newer graph was showing me. Nothing like vertical jigging, and seeing your jig in 20 FOW… at almost real-time! The only downside is the size of it.