We've had our Sleep Number bed for about 6 months and are very happy with it. You're right, it's a big purchase! But my husband and I are both plus-size and we were having trouble with traditional mattresses with the mattress sagging toward his side of the bed. Doesn't happen with the Sleep Number. He also prefers a firmer mattress than I do and the dual control lets each of us have the firmness that we prefer.
Because it's primarily filled with air it's significantly lighter than a traditional mattress, too, which makes changing sheets easier.