The excellent audio isn't of much use if the disc can't be read (poor error correction, early laser failures). My 51FD has been collecting dust for quite a while now. Initially replaced it with a Panny 110 which worked flawlessly. Just got the Oppo 105 (analog audio should be even better than the Pioneer). The 51FD was the most disappointing piece of equipment I have ever bought, and I paid full price ($550) when it first came out.
I bought a 51 soon after it was released. I finally gave up on it a year and a half later and replaced it with a Pio 320. I've been much more satisfied with the 320.
If I were buying today, I'd probably go with Oppo.