Got to say, this is something I've wondered about for a long time too. I've had a Cowon iAudio D2 for just over a year now, and it's amazing. I find it a bit disappointing that it's only got 3 stars, particularly as it's got so many things going for it - truly portable media player, great sound, excellent battery life and it's expandable (I've currently got a 32gb SDHC card in mine). The unintuituve UI doesn't exactly help though, but once you get used to it, it's pretty easy to use.
I never thought I'd find myself using an Ipod, but I got a free one last week, and I actually find that I quite like it. It's very small and easy to use, the sound is pretty good (but IMO doesn't come close to the Cowon), Itunes isn't as difficult to use as I thought, and the range of accessories certainly helps its cause. But the screen is a bit small for watching videos on.
It's handy as a second player, or to listen to when (thinking about) going to the gym, but I don't think it's quite good enough to replace my Cowon.
But you pay your money....