Agreed. Comparatively unknown compared to The National Gallery, Tate Britain and Tate Modern, but with a collection of prime items by Manet, Monet, Renoir, Degas, Cézanne, Gauguin, Van Gogh and Rousseau and a roomful of Fauves and one of German Expressionists, absolutely not to be missed!
Aside from a few high profile exhibitions, the place is always nearly empty, which makes studying the works much more pleasant (actually the main floor rooms in Tate Britain are usually nearly empty as well, though the exhibitions in the basement are often packed).
I shall be in London on Saturday to catch the Gauguin exhibition before it disappears from Tate Modern and also to see recent Bridget Rileys displayed among old masters at the National. (Love her recent work which I saw in a small exhibition at the Walker Art Gallery, Liverpool.)
Incidentally, is it time we had an art thread? This seems to be turning into one ...