I have, too, most of the time. I spent about an hour last night comparing the same songs on my iriver 140 with Xin SM3v.6/OPA627s with my iriver 120 straight out of the headphone jack. Oh, I was using Ety ER-4Ss.
The irivers have 20x20mw headphone outputs, which is pretty good. I noticed a difference in about 1/2 of the songs -- there was an increased focus and naturalness to the music, especially vocals, and a decrease in upper midrange glare, when using the amp. Is it noticeable when on the go? Not really. I'm going to try leaving the amp at home and see if I'm satisfied with the Etys straight of the 140 (which I know flies against the conventional wisdom that the ETYs require an amp).
-Erik