What does Google Sites do well? It is an extremely user friendly website creation experience. For those looking for something with extreme simplicity to convey information for company use, it is an ideal solution. I've used it in the past and I can say that the updates to the program make it even better. Well done!
What does Google Sites not do well? I think I would love templates that are a little more built out with suggestions on inner company sites so that you're not doing as much work from scratch. For someone who wants to truly customize an inner company website, they're not going to use Google. That's just not the clientele you should be reaching.
What do you wish you had known about Google Sites before using? I don't think there's anything I wish I would have known before. The premise is simple and the tutorials walk you through the basic usage, which is extremely helpful.
Have you used a competitor to Google Sites? If so, how does it compare? As I said before, other sites provide better customization options, so while Google Sites is simple, it would be better if it had sites built out for users as suggestions.