What does TweetDeck do well? This is kind of your one choice for managing Twitter. What it does well is support the official featureset of Twitter and enable you to track multiple accounts, monitor status and other things like that.
What does TweetDeck not do well? It is SLOW as in on a geologic time scale. It has no real competition so the features are implemented, but it is difficult to use. You can learn to use it, but it isn't designed or executed well to make your life better.
What do you wish you had known about TweetDeck before using? I wish there were other options, but really there are not too many that do what tweet deck does. I wish that there were better training materials because they features have been changed beyond what is available in some of the training content that exists elsewhere.
Have you used a competitor to TweetDeck ? If so, how does it compare? Sprout