Both WMP and Media Monkey are pretty easy to pick up. Media Monkey adds a lot of functionality. WMP 11 beta is very slick looking and in my opinion nicer than iTunes. Unless you have a Mac and are bound to iTunes, I wouldn't make the library software the deciding factor for buying a player. Note, the Gigabeat does not play AAC (mp4). But, provided it's not protected AAC (music bought online from the iTunes store) both iTunes and MediaMonkey can convert these tracks to mp3.
I own a Toshiba Gigabeat S. For the most part, I love it. But the Gigabeat surely isn't perfect, no device is. I haven't had problems losing music (except for one snafu with autosyncing in WMP but that was easily fixed). But I have had some complaints about battery life...i knew what i was getting into but i'm still disappointed.
All in all, the TGS sounds and looks great. The menu system is awesome. But I particularly like the dedicated buttons for volume and track selection; the Gigabeat lives in my pocket while i'm at work. However, any button press, including volume adjustment or track selection turns on the LCD which I find unnecessary. I'd still recommend the TGS. I have friends with Ipods and I don't think they compare well. I don't know any Zen owners, but i've played with the device at Circuit City and it seems nice enough (though I didn't dig the panel-like buttons).
I really want the Gigabeat S over the iPod, but after trying out WMP and Media Monkey, I don't feel I will be able to operate it. I am so much more used to using iTunes. I have also heard alot of stories about the Gigabeat S crashing or having all of the songs on it deleted frequently. Help! I really want a change, but don't want to have to deal with a problems...