Single driver speakers. Are they worth considering ?

Absolutely not. Why would you even consider? There’s a really good reason that the large majority of audiphiles buy speakers with more than one driver. It’s not dumb luck or brainwashing. It’s that multidriver speakers sound better. I know there’s a few guys that speak about a single driver that’s “full spectrum” in the hearing wavelengths that make listening to music so enjoyable. Well maybe their ears are satisfied with a single driver, but at almost 70 y/o, a musician, and a man with numerous friends in the music industry, mine are not and neither are theirs.

Personally, I am not willing to compromise. Everything that has been mentioned in this thread that I have heard has been hopelessly colored.
Some people like the sound of these speakers. So I suppose what colored loudspeaker you like is a matter of taste. 
The only type of loudspeaker that can approach full range performance and avoid coloration is an ESL which the OP did not want to hear about.
Sorry. Just the way it is. Dynamic drivers are flawed in so many ways it is remarkable that some systems can sound as good as they do. Full Range? This is an illusion.  

My experience with Omega loudspeakers was not good. Sound was too shouty, very aggressive regardless of material and tubes or ss equipment.
Dan

I am still running a pair of Cain&Cain Abby's that use a Fostex single driver and a Voigt pipe design. They have kept pace with my other upgrades, and I love them. 

Although their bass is surprisingly good, there's nothing to stop one from adding subwoofers; I also recently added rear-firing horns with exactly the aim that Ralph described in his post: "The solution is [... to] add a rear firing tweeter (and not cross over the main driver) to correct the tonality and help with soundstage palpability" (both products I purchased from Audiokinesis).

These two additions solve many of the issues with extension while preserving the coherence and liveness of single main driver. I am sure many members here have superior set-ups, but OP, you'd be surprised how full and immersive these speakers can sound.

Bache audio and Tri-Art both pursue designs built around a full-range driver supplemented by super tweeters and woofers.

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.