Most of these machines are not machines at all, but a frame that supports your head and shoulders as you do stomach crunches or curls. The curved sides roll you gently into a sitting position as your stomach muscles contract. The idea is that you take strain off your back and neck, and isolate your abs. The companies don't tell you that you also have to go on a torturous diet to erase the natural fat layer under your skin if you want that six-pack to show. They also don't explain the point of isolating the abdominals.
Isolation is great for physical therapy, but is probably pointless for most home exercise.
The abs machines eliminate most of them, especially the neck. If you're committed to having an abs machine, though, but you're put off by the price, just wait. Spring Cleanup circuit a year or two from now.