I prefer the buttonhole attachment to the built in buttonholes for most machines. I think they make a nicer buttonhole. I used my 503 and a buttonholer and did more than 100 buttonholes and buttons on that machine for curtains for our gazebo. It's possible I didn't know they were supposed to be challenging so I didn't have any trouble with mine.
Now that said, I find the zig zagger is a different beast altogether. The zig zagger leaves a messy backside. It's not a true zig zag so the stitch on the back doesn't look like a zig zag. I would use a zig zag built machine before an attachment but a buttonholer attachment any day.