Crunches have fallen out of grace in recent years, and many PTs won't recommend doing them as when performed incorrectly, they can do more harm than good, not to mention, there are better ways to get a six-pack fast.
Using an ab roller instead of doing crunches have a range of benefits. First of all, unlike crunches, ab rollers work the entire core area, including the lower back, which can help you achieve a better posture and not just a six-pack. Ab rollers also work other body parts, including the shoulder and back, so the ab rollout movement burns more calories than crunches.
It's also less likely you'll pull your neck doing ab rollouts than it is when you try to pull yourself up, holding on to your neck during crunches. That said, the ab rollout is a challenging exercise and requires some practice to master: those who carry a lot of excess fat around their torso might want to lose some of the flab first before giving ab rollers a go.
Try brisk walking or elliptical trainers first and start working on the six-pack after your body is used to exercising a bit more.