I'm a big fan of honesty. Ideally, the best balance is an honest person that knows when to keep their mouth shut. There is an art to it... but if I had to settled, I'd rather be with someone who was brutally honest then with someone who sugar coats every little thing. Fact.
Honesty is usually challenged in times of emotional charge which can be uncomfortable for everyone involved. We are either looking for a way to make others or ourselves feel better about these things that lay so awkwardly in our hearts. Such as sickness.
I came across this article titled "What to Say to Someone Who's Sick" which goes into detail about the things you should and shouldn't say.
As I've learnt in my own life, it's not about how you say what you say. Life isn't like a movie and you don't need to say things that are ornamental. What you do need to say is what's in your heart. Simple and direct. It's the rule and root to all communication.