Dont buy one unless you are a very fit and flexible person as well as motivated. I have a 6 pack and when my mum bought one of those things and i used it, it paralyzed me for about a day. She couldn't use it and my dad couldn't use it either. All they do is destroy your back. If you want abs it is painful and take a lot of motivation and effort, you cant just use a useless piece of machinery. The people you see on the adds using the machine that have abs (like mine would do the following ab workout which i do)
The main thing for a good set of abs is what i do:
The first important note is that you don't want to damage your back. When i first went to the gym i would do 200 sit ups on a roman chair (which is a declined bench) and i thought i was awesome. After about a month i had a bit of a 6 pack but i could barely bend over or stand up straight. So i got a personal trainer and we now do the following.
On the edge of your bed lean back with your feet off the bed and your back on it, and raise yourself off the bed a bit and bring your knees towards you. Your abdominal muscles are used here to balance you and stop you falling off the bed.
On the ground sit on a pillow so your tail bone wont hurt, and put your toes under the edge of your bed to keep you down. And just do about 10-15 sit ups/crunches but do them really slow. Instead of doing 200 if you do them slow your back wont hurt and it will be doing the same thing. When reaching the lower part of the extension (nearly laying flat on the ground) hold the position and you will really feel your muscles work. (my whole body shakes and it is good for the abdominals as they are holding you up)
The best thing for abs however is as follows: At most gyms there will be a bar at about your height maybe a bit taller (it might be a chin up bar or handles etc), of course you can always use monkey bars in a park as well. What you want to do is grab the bar and just dangle off. Your feet should be off the ground and sort of just hold yourself up (this will also be good for your arms) now slowly raise your knees towards your chest and then slowly let them back down and repeat. You can also raise your entire legs so that they are in line with your pelvis (so you are in a sort of L shape) however this will really hurt and is probably a bit advanced. This is good for you abdominals for the following reasons: You are holding yourself up off the ground using your arms and by raising your knees your abdominals are keeping your body from swinging and they are tense. Lifting your knees then really tenses the abdominals and releasing them back so that you are dangling extends them fully.
I also recently started laying with my legs towards one side and doing sit ups. This does your (side abs) lats but you just cant lift yourself up as high.