Ambien is absolutely a habit forming medicine. There is fairly low but possible potential for abuse (any medication that requires a DEA # has potential for abuse), but tolerance does develop commonly. When using it, you are best served by only using it 2-3 nights/wk, and then tolerance is abated.
My wife is totally hooked on Ambien. Fortunately she's on a low dose and her tolerance hasn't build up. But she's been on it for over 2 years now. After a few months it will start to wear on her, so she'll drop it for a couple days. Of course, those days off she gets hardly any sleep. She's a real type A personality though and used to keep herself up all night with a mind that just wouldn't shut down. So it's good that she's on Ambien, imho!