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).