Yeah, I'm getting some pretty impressive line out/battery times, too. Had it running for about 8 hours through the hifi, and it still had juice a-plenty to last for several 2-hour commutes and still more hours' in-office listening!
This is precisely one of the main reasons, why I bought a NW5 Walkman over a iPod, it has the best battery life. After all you want the player plugged into your ears and not into a charge point.
Of course, the sound is great and the size fantastic.
Yes,I prefer variable bitrate 192 and up, and it is in corelation with battery life but someone said that quick search with Google can solve it and increase battery life when on line-out.By the way,NW5 sounds GREAT on hi-fi and on my Sony V250 headphones.
I recomend it for everyone who enjoy music!
I find that it most definitely runs longer on line out than with headphones - I listen to it at work on line out and played it non stop for 9 hours (after a full charge), I then forgot it on just to discover it still happily playing 15 hours later when I got back to work. It then had enough life to do another 9 hours and still lasted my journey home. My files are encoded at various bitrates, but mostly varying between 132kbps and 320kbps - so it should really have lasted less than 30 hours in total.
I might be totally wrong but I think it has to do with the fact that it is insn't actually driving any headphones directly and that the stereo obviously relies on it's own amp. It is the only explanation I can think of.