Today I upgraded to IOS 8.1 with no trouble and it’s working well so far.
I am one of those users who doesn’t have the 5-GB or so of space on my iPhone to do an over-the-air upgrade. In spite of that, you can do an upgrade without needing the space if you connect your iPhone to your Mac (or Windows?) running iTunes and use iTunes to do the upgrade. It does the downloading and expansion of files on your desktop machine then installs the upgrade over the USB cable.
The upgrade/install went without any trouble and it took almost exactly 36 minutes it predicted.
I’ve not used it too much. So far, all the apps look and work the same as in IOS 7.