I've had a bit of time with my ATH-S500's over the past few days, and I am really loving them. They have a very distinctive 'fun' signature. Unfortunately I can't compare them with the JVC S500, but I do have the S400, and the AT's blow them out of the water in sound quality. The JVC's have nicer pads but that's about it. I also compared them with my Creative Aurvana Live, and the CAL's have the more sparkly treble between them, but the AT's have a more robust bass and more intimate, rich mids . The CAL also has a slightly larger soundstage and better comfort, but by no means are the AT's uncomfortable.
The only complaint I have witht the AT's is the lack of headband cushion. I don't know why any company would put out a headphone without padding on the headband. It doesn't impact on the comfort for me, but I find it looks a bit ugly.
Overall, these headphones are the real deal. They go for like $40 on CDJapan, and for that price, I'm really not sure what more you could ask for. They won't satisfy those looking for accuracy, but they are hella fun for not much cash.
Ps - The AT IM50's are pretty incredible too. Balanced, slightly warm, and very good value.
(My new loves)