I had one back in the early 70's and never had any feeding problems with it. To me it has one of the best grips and grip angles of any auto pistol. I didn't care about the limited capacity magazine, but took a little ribbing from my Browning High Power buddies. It was a great back pocket pistol and I carried it many miles hunting and fishing.
It did have a few design issues of the day that probably explains why it ended up getting sold. The sights aren't adjustable for elevation and there weren't any aftermarket sights that would cure that issue that I knew of. I didn't use the double action feature much at all. It wasn't like the double action semi autos of today since it was only double action for the first shot. Mine was long and hard to pull and I never cared for it. The single action pull wasn't terrible. The Star BM 9mm I bought to replace the S&W 39 didn't have adjustable sights either but it did have a much better trigger pull and it is a little smaller.
I still have the Star BM and it is still a good 9mm, but if I saw a S&W 39 for sale today I'd likely get it.
S&W had a drop-in elevation-adjustable sight for the 39/59s, There is a long tab on the front of the rear sight, and you just lift that up and rotate the sight 90 degrees, being careful not to lose the spring and detent for the magazine safety (or not, I removed mine), and drop in the replacement, fully-adjustable sight, It wasn't really robust, but I never broke it, either. It was a nice little sight, and only took about five seconds to replace. The DA/SA transition wasn't especially smooth, but got better when you removed the magazine safety.