Just returned from Tokyo where I was able to back-to-back compare the S400 and the S500 at www.e-earphone.jp. There is simply no contest - the S500 has better treble sparkle, better bass impact, and (slightly) better soundstage. In-store, the price difference between the two was small, about $32US vs. $40US, so it was easy to go with the S500. If you know anyone going to Tokyo, the HA-S500 has my pick as a the answer to "Want anything from Tokyo?".
Compared to other portables and IEMs discussed, I rate them pretty high as portables, not besting an MDR-ZX700 or -V6, especially in dynamic range, comfort, and soundstage, but certainly more enjoyable than the popular budget HD668b. On the IEM side, they carry a similar signature to the HA-FX40s, but with slightly better soundstage, a bit more balance and clarity. Again, not up to Sony's MH1C or XBA-3s, but very good indeed, especially for $40. At the more like $60-$80 they are going for on amazon, there's some other competition. I'm looking forward to comparing them with the CAL! some day since they're both in about the same class (compact street-looking closed cans).