Battery life was my main concern, smaller files make for more life.
IIRC FLAC is more computationally efficient to decode than MP3, so there is actually a small power saving there. In hard drive based players this is dwarfed by having to spin up the hard drive more often to fill the (fixed-length*) buffers, however in flash-based players I'm not so sure...
*32MB in my iRiver portable player. Not sure about iPods but I'm guessing around 16-32MB also.
Incidentally, I went to a talk given by Tony Andrews from
Funktion One at
PLASA yesterday and whilst his technical knowledge in the digital domain was a little lacking, the message he was trying to get across was clear: there is no place for lossy audio formats in the professional music industry.
I think (sincerely hope!) that we're all in agreement that this also holds true for critical listening situations in the consumer audio environment, and I feel that unless we are listening in a noisy environment where the signal to noise ratio encroaches on the dynamic range of the source material (in-ear-monitors such as the Shure SE530's help significantly to reduce this problem), there is no reason nowadays why we can't enjoy full-fat lossless audio goodness through a high quality set of speakers/headphones wherever we are (driving generally excepted, unless you have a new McLaren

).