What I am hearing in my test with single drive is really good. So good it triggered me to go up the ladder to a better more expensive driver, and it did get better. Single driver may not be for everyone, it does have its short comings, but what it does do sounds really wide and alive (to me). It just lacks in the lower regions, and that is to be expected. That is my, in home, listening experience.
@tunehead As long as you don't play a 'full range driver' too loud they can sound quite good.
There are two problems. The first and most obvious is bass. If there's bass excursion on the driver (meaning you are playing at a slightly higher volume) the motion of the cone to do the bass causes Doppler Effect distortion, which is readily audible as congestion.
This is why they work best at either low volume or with material that has no bass.
The second problem is beaminess, where the extreme highs can't be heard unless you are in a very specific location ('head in a vise').
So to get the most out of the driver, you'll want some form of crossover. Doing a crossover for the tweeter isn't a big deal so long as you are careful to roll off the 'full range' driver so there's no comb effect filtering interacting with the tweeter.
The bass is a different problem. If you want the most out of the driver you'll use a subwoofer (possibly powered) and then have something to roll off the bass going to the 'full range' driver. Many people like to use these drivers with SETs since they are often fairly efficient. If you do this, the best place to roll off the bass is before the amplifier so it doesn't have to make bass, which is arguably the weakest aspect of SETs since it causes them to make a lot of distortion (and they can't do full power in the bottom octave either).
But even if you have a PP amplifier, you might want to roll off the bass going into it anyway, since most 'full range' drivers don't handle much power. Bass is where the power is.
By doing this way you still get the coupling between amp and speaker that is part of the 'immediacy' that so many people describe using drivers like this with no crossover.
So if you are getting the most out of the driver, it will essentially be in a 3-way setup rather than full range with no crossover.