Ok it just arrived. Been listening for about 5 mins. now. Luckily it came partly charged.
First thoughts, wow, even after seeing all the pics and everything you're really not ready for how small it really is till you hold it in your hand. Also, it feels incredibly durable and well made. It's so small though I hope I don't lose it, I'll have to keep it hooked up at all times. The buttons are a bit iffy, but they work so that's ok.
Sonically, no hiss or interference with PK1's and Zune at all. Other than the obvious volume boost there is a jump in instrument separation, and I'm a bit of a noob at all the terminology but I think I'm right in saying there's a nice bump in all kinds of bass, and mids too, I'm not really hearing anything special in the highs department. With the bass boost on there's obvious extra bass, I'm not sure whether I'll keep it on though, the extra bass is fun, but I haven't decided yet whether it sounds a bit unnatural, it might work for some songs and not others, but it's a nice addition. On second thought, I might keep it on more often than not, the PK1's are already a nice pair of bassy earbuds and I wouldn't call myself a bass head, but I'm liking bass more and more, also I feel the bass helps with the noise isolation. Right now I have it set at a respectable listening volume and I can barely almost not hear my typing at all.
Overall, for $20 it is an obvious must buy, I'd say it's even worth twice it's amount. I like it a lot. Not sure if it makes a big difference, but I'm not using the stock IC, I'm using an IC made by a fellow head-fier, vampire copper with neutrik plugs.
Thanks Fiio for an amazing product, now I have to maybe get some double-sided tape or velcro and make a proper portable set-up.