From personal experience, they are horrible. I've only had two LG phones. With my first two (Optimus M and V), I was too new to Android to really care about updates, though I vaguely remember one of those devices being promised Gingerbread and it either not happening or it happening late and long after I moved on. My third phone was the G2x and it was promised Ice Cream Sandwich which never came. The Nexus 4 and 5 followed, but those don't count. I didn't let my experience with the G2x deter me from buying the G3, though.
While LG has been bad in the past, I don't suspect the to be horrible now. They're a rising star and phone manufacturers are now being pressured now more than ever to be quick about updates. If they're slow to update the G3, they'll lose all the momentum they're gaining against HTC and Samsung.