RE: Any negatives/positives to a STS or CSS?
The pros are that it will eliminate any remaining vibration, or shock from your bow that is not taken out by the normal accessories (Limb Savers, string silencers, stabilizer, etc.). After putting one on you may be surprised by how much vibration was there that you didn't realize you had. Another pro is that it completely deadens any sound from string oscillation allowing you to remove your string silencers and pick up a couple of FPS. They provide a consistent stopping point for the string so shot to shot consistency is improved by ensuring the nock leaves the string at the same point for each shot. Some have said this improves accuracy, but I'm not a good enough archer to notice a difference.
The cons are that it adds some weight to your bow (STS more so than CSS). They do require some adjustment to be completely effective. In some cases people have found that they did not make much of a difference on their bow. The twang that the string makes is replaced by a dull thud that bothers some people. They cost money.
I have used one for a couple of years now and from this point forward I will have one on every bow I own. I believe they are a worthwhile accessory, especially if you do a lot of shooting.