I see and hear people use that saying a lot in conversation, emails, tweets, everything. It's gotten so popular that people know that what others mean when they write "TBH". But despite it's popularity, I find it's also rather redundant. I don't think I should have to use it. Me being honest should be a given. If I'm being sarcastic, then that should be pretty clear too.

Besides, what does it really mean? I think more often than not it really means, "I'm going to share a opposing opinion to yours, and it's possibly insulting, but I'm going to try and prevent you being offended, by getting you to think 'Hey, at least they're being honest!'"

It seems like it's something that said to soften the blow. Like you're giving warning that you're going to say something to someone that they're not going to like. But not in a selfless sort of way. Actually, it more often then not comes out when someone doesn't want to try and think of way to say something with a little thought and respect, and figures prefacing it with "To be honest" somehow will make them come off a little less like a jerk.

That's my experience with it; at least how it's used today. It may have been different years ago to hear those three words put together, but like many common phrases that get abused, it now makes my eyes roll. Honestly.