When I bought my first set amplifier (Viva Solista LT) I immediately thought of trying one of those "single-driver" speakers, just to see what it was all about. I happened to see Fritz's stuff on A'gon and I really liked the simplicity of the design and the relatively low cost. My main speakers were the Opera Callas Monitors, which really sounded beautiful with the Viva, but I figured I would fool around with the single-driver speakers for a bit and put the Operas back.
I ordered a pair of the FR-6s in mahogany and I was very pleased with the quick shipping and quality of the workmanship. I put them up and listened for a couple of weeks. They sounded pretty good right out of the box, but they loosened up over the weeks. I live in an apartment and I listen at fairly low volumes, so it's important that the stuff I buy sound good when played softly. That's what I like about the Viva and Operas. I also favor simple music - guitar/bass, piano/bass, Bill Evans Trio, James Taylor, etc.
So I'm enjoying the Fritzs and after a month I decide I've fooled around enough and I put the Operas back up. I put on a solo guitar record and I am very surprised at the loss of inner detail and note-to-note dynamics the Fritzs had. I think the high-efficiency speakers are much more revealing of the note dynamics, which, once you get used to hearing, you really miss when they're not there. In any case, I now understand what the single-driver thing is about. Yes, you are giving up some tonal weight, but the dynamic shifts really do a lot in capturing the essence of a live performance.
Anyway - enough is enough. I really think the Fritzs are very well-made and sound excellent when used within their limitations. With the right ancillary gear, they can really open your ears to the nuances less efficient speakers can miss. Definitely not for everyone, but if you want to get your feet wet with this stuff, I think they're an excellent choice.
PS - my experience with the FR6 has piqued my curiosity enough to order a pair of FR4.5, with the 4-1/2" Fostex driver. The fanatics say that the 4.5" driver have even more midrange purity and their own set of virtues.