Our first call was from a user who experienced a loss of network connectivity after the update. Both PC's were Windows 10 Home (the Home version works with our software, but is generally discouraged). The update also caused at least one of the PC's to be significantly slower. This update does require a reboot and apparently in the case of this user, the update brought about a weakness in the way the two PC's had previously been configured (but which was otherwise working prior to the update). In this case it may have been a Homegroup versus Workgroup setup issue.
Problems simply in installing the update have been somewhat widely reported including being stuck and/or taking several hours to complete. Another example. Problems with respect to PC's "freezing" after installing the update have also been experienced. Prior cumulative updates have sometimes simply failed to install. This has also been the case with 4015217 update.
After installation, there have been reports of slowness and black screens.
As of April 14, 2017 there are reports of more esoteric issues such as with the VB ADODB.Recordset filter property (which a related Windows 7 update has also apparently caused). This problem does not impact our software in any way, but is nonetheless alarming. Users have had to uninstall the update to solve the recordset filter problem (however depending on your settings and particularly if you have Windows 10 Home, users will have difficulty preventing the update from trying to install itself again in the future).
While it may be too late for most, our recommendation would be, at least for now, to avoid this update if possible. Windows 10 Home users, unless they are solely connected via Wi-Fi and can set that connection to 'metered' may not be able to avoid or defer it (other than to not connect those devices to the Internet at all!).
Windows 10 Professional users may be able to prevent updates through the group policy editor (but there are indications that after this cumulative "anniversary" install, this may no longer be possible).
Some pertinent links in terms of turning off and/or managing Windows 10 updates: