I've got one. It doesn't vignette on the Summicron or the Ultron, but it does block more of the finder than the Leica vented hood that goes with the last version 50/2 and also fits the 35/2 but obviously not the Ultron.
I'd say it's a definite improvement over the Ultron's built in hood, but there are better options for the Summicron, I think. The square hood for that is brilliant.
Tom