I've tried working with plans and hit 2 problems:
Not having a planer, the thickness on plans usually isn't possible... if it calls for 5/8" 11/16" thickness I have to use 3/4" stock and change all the other measurements or it throws everything off...
I just don't have the patience required... Once the bug hits me I want to picture it in my head, draw enough of a sketch to get sizes and stuff and just go with it...
Also, I have a really bad habit of my projects evolving as I progress, with improvements and changes that I usually like in the end.. Mac