It's fast, it supports JSONP and their API team is great to work with (fast replies on email). I could recommend anyone to build a mashup on top of the Etsy API.
That's great to hear, Jonathan. I personally know two of the people on that team and they're brilliant programmers but never saw their view from the outside. I'm glad to hear you're getting amazing support from them.
If you're enjoying it a lot and love to program, you should consider applying there too.