I've had amazing results with HDR+ , especially in low light in restaurants. While everyone else is using flash, mine look natural like it was actually better lit.
I always leave HDR+ on. My last phone was an iPhone 4s. I find that the nexus 5 isn't as consistent and the camera software isn't as easy to use. However, the overall picture quality on the nexus is much better than my iPhone was. I think its just a matter of getting used to it. HDR+ on the nexus makes a huge difference compared to the HDR on my iPhone that seemed to do nothing at times.