it’s not so much a matter of good or bad but a matter of your goals. Ab machines will work those superficial muscles as rectus abdominus or external obliques, and if that’s what you want – do it.
I found that many clients benefit more from learning to properly engage the layer of muscles that is underneath, and there really aren’t any machines that do that.
Personally, I never found ab machines particularly interesting and I do not use them with clients. There are more challenging ways to work the core, and they can be a lot more fun as well.