I saw an E400 at Cranfield a year or two ago. Looked very comfy inside and with a solid build quality. It was used by some property manager who had a hired pilot flying him around Europe.
The E400 uses a very unusual water-cooled Continental engine, IIRC.
On the E500, I recall they were going to have a certified ceiling of 25k. Any plane which can reach 25k is not "underpowered".