If all I had to think about was whether or not to replace my Nexus 5, I would say no. I got the Nexus 5 on day 1, but it is still working great, and 6" is larger than what is my preference. But...my wife is using my old Nexus 4 (just like she used my old Galaxy Nexus, just like she used my old Nexus S), and it is on its last legs (has replacement screen which isn't that responsive, buttons no longer work correctly, battery life really short, etc) and she was really looking forward to getting my Nexus 5 when I purchased the Nexus 6 back when I thought it was just a normal step up from the Nexus 5 and not a phablet. I really hope that there is also an "upgraded" Nexus 5, or at least the Nexus 5 remains on sale once the Nexus 6 is released so I'd have the option to buy another one once I know exactly what the Nexus 6 has to offer.