"[The OLED is] nothing short of awe-inspiring. The first time we spotted the home screen, we couldn't help but be taken aback -- colors were dramatic, crisp and downright lovely. It really shines when viewing album art, photos and video..."
The 16GB version costs 400$ in Japan, and Engadget doesn't think the OLED display is worth the price:
"So yeah, the OLED panel itself definitely lives up to the hype, but unfortunately we're not so sure a fancy display justifies the steep increase in price versus LCD-based rivals."
I'm not sure if that's fair, because not all OLED based players are so expensive. For example the 16GB Cowon S9 Curve player costs 200$ for the 16GB version, in the US. And the OLED display is actually bigger than on the Sony player. So the high price of the Sony is not just the OLED, it's more likely the Sony brand.
UPDATE: T3 also reviews the X-series Walkman, and loves the display: "Put simply, the OLED screen here thrashes the iPod's LED one"...