In previous iterations of iOS, we have seen a slow down in terms of adoption. This is because for the past few iOS versions, Apple has seemingly allowed rather frustrating bugs to bypass its QC which has resulted in less than ideal experiences for users. Apple vowed to fix this with iOS 12 and it certainly seems to be showing.
According to data collected by Mixpanel (via MacRumors), iOS 12 has apparently been installed on 46% of iOS devices, and should very soon overtake iOS 11 which at this time of writing is installed on 46.57% of devices. The official adoption figures have yet to be released by Apple, so take this with a grain of salt for now.
However its rapid adoption is a good sign when you consider that iOS 12 was released a few weeks ago. So far reviews of the update are pretty good and it seems that Apple has lived up to their promise, which was that the update would actually improve performance on older devices in which a test found that to be true.
Of course no update is perfect, as there are some iPhone Xs users who have run into some charging issues which appear to be software related. There are also some who are experiencing LTE issues but whether or not that is a hardware or software related problem remains to be seen.