I like them because they are simple and you don't get possible artifacts from one driver crossing over to another or from driver offsets. There are a few things you need to keep in mind.
1) You are going to loose an ocatve or two off the top and bottom. No driver I know of can do the full 20Hz - 20kHz.
2) They are going to beam a bit on the high end. Again, you are using drivers bigger then most tweeters so although they can play high they will beam more. I think their "sweet spot" tends to be smaller because of this.
3) Although we are talking about single drivers there is still almost always going to be some sort of electrical network. Most (but not all) single drivers need some kind of adjust ment (filters, notches, steps, etc.) to sound proper.
Yes, I also built a pair of speakers using the Fostex FE-167E dual-concentric driver. It's a front-ported design and I am most impressed by their performance. Fortunately, I was able to use some cabinets that I already had, which was useful as my woodwork is not the best.
The speakers sound good on all kinds of music. Using the Radio Shack analogue sound meter and a suitable test CD, I was able to measure the frequency response at my normal listening position. I found that this started to fall off below 60Hz so I also built an active subwoofer, which now takes the response down to 20Hz. I'm looking forward to trying out REW on my listening room.
The thing I like most of all about the dual-concentrics is that they provide a better stereo image and more precision than the three-way floorstanding speakers that they replaced. BTW, I don't believe there is any network inside the FE-167E. They are dual-concentrics employing a whizzer cone.
I tried my hand at using a full range driver tonight...Although it's a driver not made for it. A Lambda TD15H. Mounted in a 4ft sealed enclosure, it needed a lot of eq to get to sound OK. Surprisingly the highs (above 6k) sounded good while directly on axis with nearly 15db of boost, though the 1k-3khz range needed quite a bit of help by means of cutting 12-15db with the eq.
Bass and midbass were great, though it should be since it's a 15" driver.
For a single motor full range driver, I would think beaming and IMD would be much too high for the speaker to sound decent, although it depends on output demands and sitting position I guess.