Last week Samsung launched their 55" curved OLED TV (the KN55S9C) in the US (for $9000). CNET's David Katzmaier posted a very positive review on Samsung's OLED TV, saying that the TV offers the best picture he ever saw.
David says that Samsung's OLED TV picture quality surpasses plasma (even Pioneer's legendary Kuro) and LED LCDs with no major downsides. He does say that he'd like a flat OLED even better than a curved one, though.
Indeed, regarding the curve, David says that when sitting right in front of the TV from about 7 feet (2.1 meters), the curve create a subtle trapezoid effect that was distracting and also some other distortions. It's probably something you can get used to, but still David says he'd prefer a flat TV.
David didn't yet review LG's curved OLED TV, so he can't comment on how these two OLEDs compete.