You should still work your abs and back for a strong, healthy body but you don't need special equipment to do it.
In a study commissioned by ACE, researchers found no differences in muscle activity for basic crunches, obliques and reverse curls than for exercises done on top-selling ab machines.
Conclusion: Don't waste your money. All you need is yourself and a floor (preferably clean). If you decide to get something, get an exercise ball...it's cheap, versatile and offers an excellent way to work your core.
Need some ideas? Visit my Workout Center this for a variety of ab workouts.