Have recently retired pad and pencil myself - got tired of erasing, calculating, visualising etc. To work in 3 dimensions on paper requires good spacial imagination, and has worked ok for me so far (my dual 8" pushing the limit though). I am currently learning the ins and outs of Autocad (mainly cause I picked it up cheap), and it makes things so much easier to visualise a concept, especially in determining whether something CAN be practically constructed.

Cheers
Graeme