I considered myself in good condition before I started STS, and I wanted to really kick through the plateau I was on, and it is definitely working. Cathe has done a great job with Shoulders and Back work, which was something I always thought she didn't push hard enough on in her other series. I am on week 4 of month 2, and I am noticably able to lift much heavier than I could before, and my shoulders and back are looking good.
Regarding weights... My husband bought me adjustable weights from Bowflex for Valentines day which are awesome. You can adjust in really small increments (I think 1.5 lbs) and you spin a dial to lock in the weight you want. Nothing gets easier than that! I think they were a little pricey, but totally worth it if you get them before stocking up on a ton of increments or just want to save the hassle of switching weight plates all of the time. The other thing I got are the weighted gloves she uses in the videos. These are really great just to add 1 pound on to what you normally use.