I had a fair number of rides on the 340-300 + 600, in Y and PE. Very pleasant all round and sad to see them go. The only bad point of the -600 was if you were at the back of Y. The 'stretch' meant that it could waggle. It was generally imperceptible but if you looked out for it, then a gentle movement was discernable. If I recall correctly, after early operations, there were some adjustments to reduce this in the cruise.
The VS -600, had the PE cabin right over the wing box and so it was beautifully stable.