I totally believe.
Like many people here, I've had a few experiences, which would make me mad not to believe. I don't think it means I'm mad, or have mental health issues. I just think some people see ghosts, and others don't. I think it depends on how open you are to that kind of belief.
In my old house, there was a young lady ghost who would stand by my door ever since I could remember. I never felt she was threatening- in a way, she felt protecting. It felt almost a shame to leave that house.
I've also encountered a more evil presense. It feels almost like they have control over you. It can be hard to shake off, but I believe they're also out of my life now too.
A friend of mine who died is also someone I see. That freaked me out at first, and I really did think I was hallucinating. After a conversation with another friend though, it became apparent we had both seen her. Now she visits regularly. It's nice in some ways, but also reminds me of all the fun times we had, which we'll never have again.
But anyway to sum this up, I definitely believe.