What is your take on high efficient speakers vs. low efficient speakers?

As someone above mentioned, most of these high-efficiency systems do not do really deep bass.  That is one of the tradeoffs that attend going this route.  For my taste, the very best woofers for a high efficiency system must be very agile to match the other drivers.  This typically means very light paper cones, and somewhat limited excursion (I particularly like drivers with pleated fabric or paper surrounds).  I've heard systems with twin 18" woofers per side that were still a touch limited in very deep extension, but, I really did not care much--the bass was clean, fast, and had "tone," which is more important to me.  I have no interest in a subwoofer, and several of my friends who are horn-based system experts say that subwoofers are NOT the way to go. My system has very modest twin 12" woofers per channel (pleated fabric surround, alnico magnet) in a Jensen-Onken bass reflex cabinet; it does not do deep bass.  With every system, you have to live with some limitations.

Big fan of high sensitivity speakers especially because they work well with low power SET amps.

I prefer high efficiency but I don't undersize my systems so I get bass into the 20-30hz range on most systems. I find higher eff designs to have a more realistic sound quality if done right. But many are too compromised in size. But sadly so are most of the loudspeakers offered today. Shipping costs and material costs all lead to smaller and smaller designs. I get 1 tear in my eye at audio shops it's all too small. 

The best performance comes from electrostatic speakers. They are generally mid efficiency speakers but are very amps sensitive and owner sensitive. Otherwise it is 6 of one 1/2 dozen of the other. The best performance I have heard come out of dynamic loudspeakers came from inefficient speakers driven by huge amplifiers. 

All good comments up to now. I'd only add electrical impedance is critical measure of truly efficient speakers.

As owner and modifier of Klipschorns I have had choice to continue with shallow slope crossovers or go to steep slope. Klipschorns nominal impedance with stock shallow slope crossover rated as 8 ohm, with 104db sensitivity,  it would seem to be easy to drive speaker. In reality there are drops down to 3 and 2.7 ohms with steep phase angles, requires fair amount of current. Taking this into account impacted my choice of amps, 45 and 2a3 SET were considered but eliminated due to these severe phase angles which really require amp capable of delivering some current at 4ohms. I settled on custom built 300B monoblocks which have pretty massive power supplies (each have PS equivalent to what one would normally see in stereo 300B). While I may yet try a 2a3, would have to have pretty special power supplies to cope with these phase angles.

So, for me a truly high efficient speaker is one that 45 or 2a3 flea watt amp can drive comfortably, this would be 100+db sensitivity with benign electrical impedance curve. Klipschorn or other 100+ sensitivity with less benign impedance curve something slightly less than truly high efficient.