As I am limited for time off the shelf has the advantage, quick and easy. Years ago I built kits but with the super detailing detailing of today's models surpassing the detailing of kit models then for me off the self it is. No more painting, spending hours fitting a part just right. Now it's take it out of the box adn you're ready to go.
Tri-ang did CKD kits which you could put together using a screwdriver and nowt else as a rule, sadly these are long gone although they turn up on EBay from time to time.
