Success for me: Clover, X99M Extreme4, 3.3GHz i7-5820K, Asus Strix GTX 970 OC 4GB, iMac 14,2 SMBIOS, upgrade via App Store from 10.10.3
Whenever I lose the NVidia web drivers I have to install my backup graphics card: EVGA 8400 GS and choose to Inject NVidia from the Clover graphics options. Boot up, install web drivers, shut down, replace graphics card and boot as normal.
Still can't get iMessage back to a working state, but that broke before I upgraded (Contacts on iMessage show up as red, i.e. is not registered for iMessage. I can send as SMS by linking with my iPhone). No idea what stopped it working.