I went from a Iriver H120 to 4G Ipod (U2) - I also owned the original Creative Zen
It wasn't my choice to switch players but it was given by my bro and sis as a christmas present so I didn't want to be ungrateful. Anyway, after using it for a couple of months I have fallen in love with Apple's UI/Click Wheel. There's nothing hard about navigating on a Iriver, especially since I was using a similar folder structure as the one on my PC. What I didn't like is using the joystick to get from one point to the next and how slow it was in doing it. On the Ipod, though, the click wheel makes things so much easier and quicker to get around. Plus, I love how I can easily scan through a track to a later time in the track (I listen to mp3s that tend to be 1hr + - live club mixes) by using that diamond option. With the Iriver, I would cut the battery life by a significant amount if I had to skip through a track by using one of the options that skipped to the next 7 minutes when forwarding in a track.
I also want to make a comment about the sound quality. I was really concerned how my mp3s would sound like after hearing how much the Ipod sucks in this area. I've come to realize that most of the comments that I read were either from people who were extremely anal about sound quality or made comparisons using Apple's stock earbuds. I'm using the same earbuds I used on my Iriver (Sony Fontopia) as I'm using now with my Ipod and I honestly can't tell difference. THere is nothing flat about the sound and I would definitely notice if it was since most of my music is bass-heavy.
But yes, Ipod lacks the features that other players do provide out of the box. I kinda miss having a FM tuner and Voice recorder builtin but I never really used either so I can live without it.
So would I go back to a Iriver player after I retire my Ipod? Probably not. I don't care for Iriver's crappy support.