I owned a share of a Diamant 18 from 1974 to 1979.
It was my first glass glider and I found it relatively
easy to fly with few vices. There was a mod to the
all moving elevator that some UK Diamants had -- ours
did not. The mod was a small extension to the middle
aft part of the elevator and was intended to increase
the pitch stability. Without this mod the pitch stability
is rather poor and in fact stick forces REVERSE at
high speeds. To make this clear, the faster you fly,
the more you need to hold the stick back! Very weird.
If you let go the glider would (presumably) go into
an immediate bunt.

However we never bothered to have the mod done and
I guess that shows that we did not have any qualms
about the handling (or were we young and foolish?).

As others have said the brakes are excellent and extend
from the top and bottom wing surfaces.

The canopy opening mechanism and cockpit shape are
very unconventional but caused us no problem. A previous
owner did suffer a partial opening of the canopy in
flight and flew the rest of the flight one handed while
holding the canopy on with the other!

I don't remember any problems with the rectractable
U/C. However I did manage to bend the axle in a firm
landing. So I would say that the U/C is strong enough.

Ours was landed wheels up at least twice. Once on
a tarmac runway with a cine camera strapped to the
underside. This caused a fair amount of damage. The
second time I was the pilot and got away without any
damage on a soft hay field.

I don't think I have flown a better glider for weak
wave. However a modern standard class glider will
outperform the Diamant in thermal cross-country tasks.