I haven't compared them side by side, but my opinion, as well as info that i've read, is that iriver made a big attempt to make a worthy headphone output. The output is higher wattage than most mp3 players, 12mW instead of the usual 7mW. Of course, its still just a portable player amp, but apparently its quite good, as far as portable player amps go.
I like that there isn't an LCD on the unit, i'd rather save space and weight on the main unit, and just have it on the remote. and to those who gripe about losing the remote being a big drawback, just don't lose it! You could say the same thing about the headphones 'man, this player sucks, all you have to do is lose the headphones, and you can't use it anymore!' if the remote is always going to be attached to the unit, there wouldn't be a reason for it to be put elsewhere, and would be hard to lose. You'd either lose the whole thing, remote, unit and all, or they'd stay together.
Given a choice (oh wait, i had a choice
) I'd choose the slimx (and i did). I think its a better designed player, with more intuitive and useful features, and its got good output stages, as far as portables go.
peace,
phidauex