Lemme start with the first one that I've ever bought, and will never by again. Mind you, this was around 2005 A 128 Meg Rio Forge. I picked this model as it had a SD Slot and would work with Audible.com's DRM'd audio books. I bought 2 of them, both of them craped out after a month or so of what I'd consider normal use. The second one was a refurbished model, so after the luck with it, I moved on to Creative Lab's MUVO models. I will NEVER recommend one of these or purchase another RIO product.
I LOVE the Creative Labs MUVO's. Sturdy, and reliable. I've I expect that after a over a years worth of serious use, I've gotten my money's worth. I have had the 128 Meg, 512 Meg, and 1 Gig models. None of these have an external drive slot. But will work well with with Audibles DRM, but no joy in hacking their Firmware. Other the the lack of OGG support I'd HIGHLY recommend any of the Creative Lab's products. I've thought about getting a Zen. One of the little nuances that one finds with listening to a Podcast or Book is that you want your player to start up right were you left off if it powers down, or you turn it off. One of the downsides of a Muvo is that if you've been listening with it for 4 hours straight, and the battery goes dead while you're listening to it, it will go back to the last place saved. Eh, I can live with that. Powered via a AAA battery. A rechargeable or Cheap-o battery lasts about 8 hours of use, a New Duracell will last closer to 12 of constant use. So if you're planning on taking one camping, keep this in mind.
SANSA: I have two Sansa e250's one I Rock boxed, and when trying to update the version of Rock Box I ran into a snag, so I went to completely remove Rock Box and ended up bricking the dang thing. I should have left well enough alone. But I'm a hacker and tinkerer. Do I Blame Sanasa for this? Nah. They weren't the one's messing around with it. I was going to do something fancy with a second e250 that I had bought, but since I bricked the first one, and until I find out where I can get the e200tool, as it's supposed to help un brick the sucker. Until then, I'm going to hang out with the standard interface on the other one that I have. But up until the time that I started mucking around with Rock Box on the 1st Sansa e250 it was working ok. Battery was draining a little faster then I expected, but oh well. But these charge off the USB port, and charge should last at least a day of normal use.
I also have a Sansa e140 and it's ok, and but it does one thing that I simply hate, and won't keep it around for podcasts or books. It's fine for music, as it won't bookmark your current spot in a track. It runs off a AAA battery.
Part of the ensemble on my dashboard. (Over the road trucking will cause this) Is a VR3 FM modulator. Works good, can get blasted from outside FM signals depending on where you are, but you can move them around with the presets. Great if all you want to do is listen to music, as there isn't any bookmarking, if you loose power to the Cigarette lighter, it goes back to track 0. It works good to plug a USB cord into (say a Sansa e250) to charge you MP3 player. It also will play the non DRM tracks off of the Muvo if you plug it into it also. But it just reads the Muvo as a standard USB flash drive.