My turn to weigh in. Upon the advice of my dealer, after reading all of the above, I nonetheless chose to trade in my 390S for a 512, which I got last week. I had some inital misgivings prior to making the decision, because I had been generally happy with the 390S since 2003, but my dealer assured me that the improvement would be substantial, and that the reliability issues had been solved.
I can't comment on the reliability issue yet, but the sound is in a different league than the 390S. The first day I got it I listened to old familiar CD's (don't have any SACD's yet) for several hours. While still lacking some of the sheer emotional energy of a very good turntable and LP, the player has absolutely none of the negatives I have come to associate with digital. Compared to the 390S, it is able to resolve even the most complex, loudest passages without a trace of strain, harshness, or muddiness, something I've never heard from digital. It doesn't "reach out and grab you" immediately, but after a few minutes of listening, I inevitable find myself thinking "I've never enjoyed digital this much!" Even after several hours of listening I have no desire to switch to records, because while a great record would sound better in my system, this player makes an average CD sound better than an average record. Not really surprising, I guess, given that the 512 is over 5 years more current, and twice as expensive.