What is your opinion of single driver speakers

great for headphones.

I built a pair of those using a Fostex driver and they were pretty darn good. That Pearl Acoustic speaker is based off the Mark Audio Pencil speaker which has been around a while. 

I wouldn't use them in a big room nor is it "loud and proud" type of speaker.

Single-driver speakers can be dynamic and sound good however they really need one or two separate powered subwoofers as most multiple speakers need.

I've heard only a few truly single driver speakers whose overall performance was pretty good with all types of music.  One is the Charney Companion speaker with the AER driver, the other is a Voxativ speaker.  I also recently heard a Songer single driver system that sounded very good, but, I did not hear them enough to decide whether the Songer single driver or Songer full-range with woofer model was better (amazingly, they are the same price). 

But, I've heard many truly excellent systems that used full-range drivers in multiway systems.  I recently heard a Voxativ two-way that sounded really good, I like the Cube Audio Nenuphar Basis (10" full-range plus a built in subwoofer), and I like the bid Dolce Vita system with an 8" full-range driver plus a tweeter.  I've heard a few custom-made systems that also ultize a full range driver with a tweeter or woofer, and liked them too.