Mick,
A friend of mine Russell T. Has one of Marks mirrors.
A 10" F6.3 I think. Its just over F6 anyway's.
I have looked through so many scopes its not funny.
But this scope is as good a scope or mirror that I have seen.
Without going into a lot of detail. We have observed Saturn in good seeing at 500x I can't describe how good it was.
We have observed the Moon also at 1450x I'm not kidding.
No image break down at all. Clear sharp view.
gts055 (Mark) will back me up on that.
Now if this mirror is one off I don't know.
But it gives a good as it gets star test.
I had a GSO 10" great scope for the price.
But it was not upto Russell's scope.
So yes I think Marks mirrors are worth it. IMO.