I always thought the PDQ-2 was a cute and simple airplane. I only saw one completed and flown, probably in the late seventies or early eighties, but since have not seen any. The only comment I heard from anyone as to why more wern't built was because of the lack of engines. The airplane came out well before the ultralight industry evolved and as such, there were only very few options of what to stick behind the pilot.
Today though there are may more engine options so I would think it would be a much more realistic airplane to build and fly. Personally, I liked it because it reputedly handled more like a normal airplane than a lightly loaded ultralight. I've never flown one so I can't tell you first hand though.
The plans are still available http://www.sirius-aviation.com/pdq.html
Personally, I wouldn’t fly something like that, no pilot protection and in a crash the engine will come right down on top of you, few years ago there was a new plane design introduced at Sun”N”Fun that looked just like the PDQ (Cant remember the name of it now) but made mostly of composites, the fuselage was still aluminum like PDQ and it had all fiberglass wing and fiberglass cockpit pod which was optional, The airplane crashed on take off and the engine mast collapsed as I predicted it would, the engine smashed into the pilots chest and killed him. That was the end of that project.