Single driver speakers. Are they worth considering ?

Of all the true single-driver systems I've heard, the quarter wave back loaded horn systems of Voxativ and Charney are the only ones I really liked because they had reasonably extended frequency ranges and smooth response (not excessively peaky and rough sounding).  I am particularly fond of Charney speakers with the AER full-range drivers.  The Cube Nenuphar was okay too, but, its frequency response was not quite as smooth and natural as the other two and it lacked the upper bass weight needed for large-scale classical music. 

I don't consider coaxial drivers to be full-range drivers, they are two drivers built into the same frame and utilize a crossover like any other two way speakers.  But, they certainly can sound good.  The big Tannoy systems, like the Westminster, are very good sounding.  

I have heard a lot of fine sounding multi-way systems that utilize drivers intended for use in full-range systems as either a mid-range driver or the bass/midrange driver with a tweeter added on top.  I particularly like the Jensen M10 field-coil driver used as a bass/midrange driver with a tweeter crossed in at a very high frequency.

The comment above about full-range drivers having "perfect" dispersion makes no sense to me (I have no idea what perfect dispersion even means, never mind how any system can achieve it).  Given that most full-range drivers are relatively small in diameter for a woofer, they have WIDE dispersion at lower frequencies, but, given that they are large in size for a high frequency driver, they have a very narrow high frequency dispersion and need to utilize things such as a whizzer cone to attempt to spread highs adequately.  What they more or less avoid, as compared to multi-way systems, is comb-filter problems of two or more drivers interfering with each other.  

Erik,

I've heard a lot of wide-band/full range drivers used very successfully in multiway systems.  Some were, as you discuss, assisted by a woofer, others were assisted by a tweeter (most commonly with large diameter full range drivers) and some with both a woofer and tweeter.  Most of these were custom-built systems.  An example of use of extended range drivers in multi-way systems includes the SoundKaos Model 42.  By using simple, first order crossovers and allowing the crossover points to be away from the critical midrange, fullrange drivers minimize the negative aspects of multi-way designs. 

dht4me,

I have not heard the AER BD5 driver, but I would bet that it sounds terrific.  I've heard the BD2 utilized full range (no other drivers) in a couple of systems.  One just had a large horn on the front, and was completely open in the back. This system had a very pure sound, but no deep bass.  The other was a Charney speaker that had substantially more bass because of its use of a backloaded horn on that driver.  The Charney speaker is fantastic.  I've been somewhat interested in the BD5 in a backloaded horn system.  AER sells a very expensive system that utilizes that driver in with a front horn/waveguide and a backloaded horn for bass.  It is an interesting looking system, but, it is quite expensive.

I've also heard, and liked, a full range Feastrix field coil driver.  The system was a bit deficient in bass, but, the driver was in a fairly small Jensen Onken cabinet, so I don't know if bass response was optimized.

I would love to hear the Shindo Lafite.  I like the Latour (two-way field coil Shindo system).  I really like field coil drivers--they sound very dynamic and "alive."

While I would be totally satisfied with a Charney full range system, and perhaps the Voxativ back loaded full range system, I also think iopscrl (above) is correct that you get much of the advantages of such drivers while ameliorating the limitations of trying to deliver all the sound from one driver by using such drivers to cover a wide midrange.  I have heard the Jensen M-10 (13" field coil) with a tweeter, and the GIP Laboratory 4165 (12" field coil) with a tweeter to deliver the kind of dynamics and clarity that full range systems are known for, while being much smoother sounding and extended at frequency extremes.  

I have heard Western Electric 755 drivers used in single driver systems and in systems with a tweeter on top (only crossover is a first order high pass to the tweeter, the 755 runs full range) and much prefer having a tweeter in the system.  I think systems using the Western Electric 756 and 728 with tweeters are even better.