How important is the efficiency of a speaker to you?

More Efficient just means louder. Alone it has little to do with what amps will drive the speakers well ie produce the intended sound completely. That has more to do with how difficult a load which is more about impedance, not efficiency.

So really all more efficient means is a flea powered tube amp can make them go louder before clipping sets in. It does not mean one is getting all they can out of the speakers regardless of amp. That’s not to say the results will necessarily sound bad, just not optimal, and different with different amps, not the same

Take Fritz speakers as an example. If you check out the website you find these are designed to be an easy load for any amp but are not high efficiency. They will sound more similarly good off most any amp and still achieve decent SPL with most any amp, so they are an excellent choice for someone who wants to get best end results using most any amp. The site also shows how the Fritz speakers compare In this regard to other popular speakers. 

Efficient also means the drivers don't have to work as hard to make music...It's been scientifically proven that efficient drivers are happier. Also factually true is a 500 watt or so amp into efficient speakers might be using 1% of its mojo. And yes, the correct term here is "mojo."

My thinking is that efficiency is a product of both reactance (impedance and phase angle) and sensitivity. The reactance relates most toward 'easy to drive' while sensitivity relates more toward size of room to be pressurized. 

Since the Fritz speaker has been mentioned, I will add that Fritz speakers are easy to drive having a benign reactivity. A sensitivity around 88-90dB allows them to be played in smaller rooms with low wattage amps. As room size increases power requirement also increases however I can't imagine anyone needing more than 100wpc to drive them to their maximun output. If one is playing them that loud their room size/listening level likely is better suited toward a larger speaker.

I prefer speakers with a sensitivity range between 86-88 dB; I dislike those with a sensitivity higher than 88 dB because I want to avoid any hiss from the speakers. Contrary to the belief that high-sensitivity speakers perform better in low-level listening, the key lies in how you adjust the equalization to achieve a proper ELC.

Another misconception is that people believe high-sensitivity speakers can be driven by lower-wattage amplification and produce equally good, if not better, sound quality. There are a couple of reviews demonstrating that the Klipsch Cornwall IV (102 dB) can be driven with a higher-power amplifier to achieve better sound quality with more control throughout the frequency range than lower-power counterparts.

I went from 87 db speakers to 94 db speakers. Much better sound at low volume and less power needed.