Bidding far in advance is a risky business because you can't cancel or change the way you can with normal reservations. You are paying no matter what happens. The shorter the span, the smaller the risk. Often hotels will have an option for a nonrefundable low rate if you do it far in advance - but the risks are the same as with Priceline. The best deal is the one you get the night before.
Totally depends. When I have fixed plans to go to a city, it extremely rarely changes. Pretty much the only reason is sickness, which would be covered by my annual travel insurance policy but fortunately has never happened yet.
I have booked hundreds of room nights with priceline and have used every one of them.
I already have trips planned for next June and would have no problem at all booking hotel rooms through priceline for these dates if the price were right.
I have extremely rarely seen non-refundable rates as low as priceline rates. I have also rarely seen non-published rates like government or industry rates as cheap as priceline rates.