I don't have a 659 but a 559. Kinda the same thing but not so shiny. Back in the day I had to have an "all steel 59" so it joined my herd. Yeah it's heavy when loaded. It's been completely reliable except when a magazine spring got weak and the slide didn't lock back. Easy to fix. I put an arched backstrap on it which completely changed the feel. I never took a liking to the flat one. With the arched backstrap it feels a lot like the 39 pistols. I guess I'm an anachronism, but I like steel pistols. Many of us here are anachronisms, I figure.
I wasn't aware there is an arched backstrap for these guns. I know the 5906 and similar have flat or arched grips that cover the mainspring. I would like to find one of those. I've always liked the arched mainspring on a 1911, and I like the 39 with its arched grip also.
The 559s are gorgeous guns actually, I've never run across one however. I'm an anachronism also. I hardly ever even shoot autos, I'm stuck way back in the past with my "wheel" guns...