What does ShareThis do well? Amazing at consolidating all the social share icons into one clean user friendly icon. Great for webmasters that are concerned with website aesthetics and functionality. Highly recommended option if you don't want Twitter, Facebook, etc. icons all over your content creating clutter. WIth ShareThis you could have every social platform contained within so that you don't have to limit how many you want to include because of clutter.
What does ShareThis not do well? To be honest, I can't think of anything that it doesn't do well. I don't have experience with competitors to know if there are better options out there. I even find the coding quite easy to implement so I can't even say that they don't do that well.
What do you wish you had known about ShareThis before using? Knowing that there are some custom elements that could be included would of been a plus knowing beforehand. Some of the coding can be dynamically setup as well for ease of implementation.