can you keep some bitcoin "unconverted" on their exchange and decide to convert only when you need it?
Yes. Just login to the exchange and use the address shown there.
great news!
So I can exchange BTC on rate shown in order book, and then i have immediately available on debit card balance?
Yes. However i havent seen any trustworthy words on mistertango working yet, nor if they were a member of west crest.
I ordered this card a week ago, and i havent heard ANYTHING since. If they really work, card delivery is probably about 3-6 months because they get like 1k+ new memebers per day.