Hiya Richard, they're actually pretty good. The first time using it it's a bit fiddly and you kind of get it wrong but after a while it becomes quite intuitive. The results are repeatable so it's definitely accurate. You do need level ground and at least 1 meter either side. Are you still in Lanark? I can bring it over some time for a demo if you'd like.
The other thing I like about the trackace is that the guy that created it just developed it in his shed.