I've had mine altered extensively. It was simple, even though it was quilted as well. It hasn't affected the waterproofing at all, I've worn it quite a bit. I've had the sleeves narrowed slightly, the waist taken in, and I've had the crappy paper faux leather looking buttons swapped out for some beautiful natural horn ones. It's one of my favorite casual coats. I guess you could call it a barn coat? I don't think I paid much more than $60 for everything alterations wise, and I'm very happy with the results. Although, I can't layer the coat much since it's slimmer now.
I had the overall length shortened. I needed a much larger size to have the sleeve and shoulder length I needed, being 6'3", but it was too wide and too long. I had a tailor shape a nice lower quarter that ran right along the bottom of the the front pocket making it the right length for me. Plus I had him close the rear center vent, I didn't like how it seemed to bell out, and honestly, it felt very drafty with it open so high in the back.
The sleeves were really big and loose originally.
The natural horn buttons add quite a bit to the look without looking out of place.
I had the buttons moved slightly over, to make a snugger fit, since I only wear the jacket over thin cotton shirts or lightweight sweatshirts.
On the original design, the bottom of the coat extended a good 3.5+ inches below the bottom of the pocket. Getting the size I needed for shoulder and sleeve length ended up leaving me with a coat that extended so far below my ass that it would get caught up when I sat down.