We have a couple of the 1 gig M240s.
Do we like 'em? Well, I'm about to upgrade to the four gig unit. What do you think? ;)
One of the main reasons I went for 'em is because they DO use inexpensive, easily obtainable and easily replaceable AAA batteries. I get about 15 - 20 hours on an alkaline AAA. Figure on considerably less if you use rechargeables. When I get the "low battery" message I simply pop in a new AAA. I'm good to go in less than a minute (assuming you keep a spare handy).
I see kids at school with ipods and when they run out of juice they simply stop and have to run to their charger and wait for it to charger up. Not a biggie, but a bit of a pain.
Sound wise, I have 243 songs (wma) on my one gig unit and the sound through my Koss Porta Pro headphones is excellent. When I play it through a big stereo, the highs are slightly, just slightly muted.
The FM is about as good as it gets on a portable but a long headphone lead helps.