Well, last winter I made a couple of jackets--a windshirt and a gore-tex shell, and made them both as simple as I could to save weight. No hand pockets, pullover style with a zip at the neck, and the gore-tex jacket has a hood. I had never made anything that complex before, but I didn't have a pattern either.
I just took clothing I already liked and copied it--laid out a shirt which fit me well and traced a pattern on some cotton fabric (adding seam allowances) for the basic lines, added what I wanted, and then put that together as a mock-up. Both jackets worked out pretty well--very functional. I found the most difficult part was getting the right angle for the underarm to get the armpit to go together correctly. Ended up having to add a little patch of fabric on the gore-tex jacket, so now I guess I have gusseted underarms or something!
All in all, I think you're actually better off with a pattern if you can find one which suits you, but it is possible to make your own if you are so inclined.