Sandisk just came out this month with their new flash drive based 512 & 1gig Sansa line which includes an SD expansion slot. I've read good things about their MP3 players (the flash based version w/out the expansion slot) and that they run for up to 15 hrs on a single AAA battery. The new Sansa line claims to have the same battery life and boasts of an FM radio as well. The new line is selling as low as $134 for the 1 g version (although list price is much higher). Personal online reviews aren't abundant yet as it's too new, but I saw one where someone mentioned his battery life was not as expected (possibly due to running the files off the expansion card??).

I myself am in the pursuit of a new mp3 player. I put it on hold after I got my Treo phone as I can listen to mp3's on that with the 1 gig expansion card, BUT I dont feel at all comfortable using that device (Treo) while exercising or running, although as a mp3 it works great, still I wouldn't like it if I dropped it while exercising , so I dont use it for that.