Use this phrase when you're not really sure that you should share an opinion, but you share it anyway:

To be honest, I don't really agree with the direction the company is going in these days.

You can also include words like "completely" and "perfectly":

To be completely honest... I think that he's a creep and he's just using her for her money.

You can also put "to be honest" at the end of a sentence:

I've never really liked action movies, to be perfectly honest.