My release day iPhone 4 (ordered from Apple.com) was fine for the first couple of weeks and then the battery would drain by up to 30% overnight.
So it got replaced.
My Replacement was fine for the first week or so until I installed update 4.0.1 and then it would display No Service and it would constantly lock up and force me restore the phone, but it would always revert back to No Service after a few minutes of use.
So it got replaced.
My current iPhone 4 is fine, but the battery doesn't seem to last as long as my 2nd iPhone. I've made it through a couple of weeks and all is well, but I refuse to install any OS updates. I'm still gun shy.