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.
I prefer more functional movements than the typical isolated movements that machines limit you to. Especially if you are not a beginner or someone with some kind of an injury or other limitation, I would stay away from them as much as possible. There are so many other variations of abdominal exercises which you can perform that make machines obsolete. If you are looking for more resistance while doing your abdominal moves, use a medicine ball, kettlebell(s), DBs, sandbags, free weights, etc. Also Turkish get-ups are great for core stabilization and strength (among other things).