What is your opinion of single driver speakers

One sided

Single driver speakers can be quite nice in the right conditions.  Think small monitor speakers.  Can't expect room shuddering bass or loud listening.  But they typically have GREAT TIMING and IMAGING.  Good dynamics.  BEST in nearfield / small room or nearfield far from room boundaries.  Cone breakup can be problematic in some drivers.

I have tried several systems using Mark Audio drivers in Pensil enclosures and CSS drivers in other arrangements.  

I run Konus audio, Essence speaker's.

They use the Jordan JX92 driver's, I absolutely love them. Fast, detailed, superb imagery, and good to look at, (which means the wife likes them.

They are compact for a floor standing design, quite wide but the cabinet is just 4" deep, so very room friendly. 

Bass from the 4" driver with its folded labyrinth is plenty for my needs, but my listening room is only small. 11' x 11', so may be an issue in larger rooms

In some respects, co-axial driver speakers, like Tannoys, have similar sound as systems employing full/wide range drivers—a vivid, lively sound.  But, most full/wide range speakers have higher efficiency, making them better suited for use with low-powered tube amps (the best sounding amps), and tend to be better at low volume.  Still, a Fyne F 1 coaxial is a speaker worthy of serious consideration.

I have Concentric drivers in my Tannoys and they are remarkable. The sound is from a single point source which producing amazing soundstage.