Rumor is that the seemless upgrade is automagic from the phone owner's perspective with no option to opt out.
I buy only Nexus devices so I get updates without handset maker interference, but I still only want them when I approve the installs.
I also want CHANGELOGs! It would be nice to know what's being installed. I'm fine with the high level description, but there should be a way to easily dive down into more information than "Bug fixes and performance improvements.".
As examples on the wait until I'm ready: I recently had a system update come through when I was off-continent and not in a position to muck about if the update hosed my phone or even just my settings; there was also a phone app update as I was negotiating an expensive contract with some impromptu requirements, so the update waited until everything was signed and I was certain the money was coming in.
I also generally ignore updates that require GPS access :).