Abdominal exercise devices are many in number and range form simple inflatable balls to complicated structures with springs hinges and hydrolic resistance mechanism.
The Exercise equipment industry also recognises the the motivation for many people to exercise is to reduce a growing layer of fat around the mid-section, so an obvious choice for any one looking to loose weight in this area would choose a device to directly taget this area.
Althogh this strategy seems to be logical, the effectiveness of an abdominal exercise device on the abdominal area is much less than one would think, in fact the effects of abdominal exercises are far less than that of walking, jogging or cycling for a similar level of perceved effort. The fact that most abdominal exercise devices use a muscle isolation form, will give a far greater feeling of a good workout because the lactic accid build up during exercise causes some discomfort which is normaly ascociated with vigerous exercise and thus giving a percieved level of exercise which has actually not depleted the normal level of blood sugar in the bodies energy system and thus not effected the bodies energy system to break down stored fat into usable energy.
Many people also use fitness product purchases as a means of motivation for a healthy lifestyle. I thin every one at some point in there life has bought something as a means of motivation a lifestyle change, when in actual fact these changes are usually short lived and have very litle effect. My advice would be to change your lifestyle then buy devices or gadgets to re-enforce the changes.
If you are still determined that an abdominal device is the right choice I would recoment a small device that doesnt take up half you house, unless you have a spare room, some thing like the 6 Second abs or the Ab King Pro will certainly give you a good workout.