I have tried AirplanePDQ, though it's been a few years since I played around with it. And I haven't done anything real extensive with it. I'm not any kind of aeronatical engineer. More of a wanna-be airplane designer. I do have a fair amount of CAD experience, however. For nearly 20 years my day job has been as a CAD Designer for a (as they say on Wheel of Fortune) "major custom rubber molder". I have used a number of CAD programs, including AutoCAD, Solidworks, and KeyCreator (formerly known as Cadkey).
PDQ was developed using the IntelliCAD (http://www.intellicad.org/) platform which is, basically, an AutoCAD clone. PDQ then adds aircraft performance (weight/balance, Vx speeds, etc.) analysis to it. You key in the aircraft parameters (i.e the type of plane, number of passengers, engine type, etc.) and performace figures you're looking for, and the software configures the plane.
It will then produce all kinds of charts and figures, some of which I question. For example; the weight & balance. I don't now how it can calculate those figures without knowing what materials (and associated weights) were used to build all of the different components in the airplane. And where they are located. And I haven't seen where this data is input. Though maybe it's there and I, in my limited playing around, just never came across it.
Anyway, in my view, it's only good for estimating "preliminary" numbers. Which is fine if you understand that. And for the price, and what you intend to do with it, that might be good enough.
I've never tried OAD (didn't even know about it), but now that I do, I'm going to download it and give it a shot.