Are you asking how useful fingerprinting is, or are you asking how effective it is?
For the former, any amount of fingerprinting lets ad companies make the case that their ads are better or worth more money. For the latter, the EFF Panopticlick research project seems to be pretty good at giving you an idea:
I don't have an iPad/iPhone etc. to test Safari with; seems like they'd be amongst the most homogeneous of devices. Perhaps someone with one of those devices could tell us if the EFF can uniquely identify their device. It was able to uniquely identify all of my devices.