I just bought a new (California version) Shield. At 73, I was having some difficulty locking the slide back, particularly when reassembling the pistol after field stripping. Like many others have stated, I had to push the slide against the edge of the workbench to lock it back during reassembly.
Based on a number of online reviews, I purchased a SS RSA, which I received a couple of days ago. I installed it, and instantly I was dealing with a different pistol. Not only was it easy to lock the slide back, but I was also easily able to do so when reassembling the pistol. In addition, the slide moved more smoothly with the single-spring SS RSA than with the stock dual-spring RSA.
This morning I put 150 rounds through the pistol at the range without a hiccup. For me, adding the SS RSA, the Apex trigger kit and Trijicon HD night sights has turned a very good pistol into a great pistol. Your mileage may (and probably will) vary.