I had a very rocky experience with iCloud Photo Library in late 2015:
I imported my photos and uploaded my 70GB photo library to iCloud. Everything went very smoothly, so I turned on iCloud Photo Library on my iPad.
Aaaaaand it all exploded.
Apple Support eventually got me up and running, but it took a while for me to fully trust the system.
The reality is that iCloud Photo Library is pretty great when everything is working smoothly.1 Photos show up on my devices quickly, regardless if I snapped them with my iPhone or imported them from an SD card on my iMac.