Amazon is usually last to ship, B&H or Adorama are first. They will not cancel your order at the drop of a hat either.
Amazon is great, I've never not been able to instantly cancel a preorder that is still waiting for the product to arrive.
The issue is that Amazon cancels pre-orders without asking you. You might be waiting 3 days or 3 weeks, and suddenly, your pre-order is cancelled by them. Usually it happens due to simple things like typo's in the listing, but they may also cancel if the order is not filled within 30 days, that seems to be sporadic and is based on consumer laws requiring shipping within 30 days or a confirmation by the customer.
It is extremely frustrating to get on a pre-order list on the first day, and then 3 weeks or more later have your pre-order cancelled and, in effect, being told to go to the back of the line. In my case, I then went to Adorama and there was no problem getting my 7D in short order, in fact, before Amazon started shipping them.
I do use Amazon Prime, and its nice. They have a big advantage in that they have warehouses all over the country, this means that there is a warehouse two days or less by ground to you. Thus, they can negotiate two day shipping with fedex and UPS, or send by USPS and the shipment will not cost them more than ground shipping. If they are out of stock, then it comes by air from further away.
Adorama and B&H have only the New York shipping location, so they pay more for shipping to customers that are far away.
Another thing to consider is the complexity that Amazon has with very expensive inventory that is prone to pilfeage and getting new products safely to their many warehouses in a timely manner as well as keeping track of all of it. Its much simpler and more reliable to have it all in one place, and even then, errors in inventory count happen.