Has anyone had any experience with handheld units? I've been looking to pick one up, but I'd like to know the difference between them. What do you get with a 120 dollar one that you don't get with a 45 dollar one? I just want to use it for checking depth before I drill. I don't just want to keep drilling holes if I don't have to. I have an old flasher and I've tried to use that, but just get wild readings. Any hints or insight?