Unless you are getting both consoles, I don't get why people would pre-order prior to E3. I can understand pre-ordering generally, it's a good way to guarantee a console on launch day, but not waiting until we know all the facts about both the consoles, know more about the launch lineup and have seen some actual gameplay, that seems crazy to me.
On the PS4 side, we don't even know what it looks like yet and don't really know much about the launch lineup; and on the Xbox side, there's been no actual gameplay footage yet, and again we don't know the launch lineup. I can understand that it's easy enough to cancel or change the pre-order, but E3 is only a few weeks away now, surely it's more sensible to wait until after that and then pre-order?!