They worked pretty great (F model, no video playback capability). I replaced the firmware on mine with Rockbox. To my untrained ear, music from the Gigabeat was better than the iPod (not so much after replacing the firmware, though). As for battery life, it lasted much longer than my brother's 5th gen iPod. 20~24 hours of straight music playback, iirc.
Don't use it all that often now, since I have an iPhone. Convenience trumps audio quality. Now, I just use the Gigabeats as external USB hard drives.