A BOSU is a great piece of equipment that could help you increase your core strength, but is is very pricey. A new BOSU will set you back $125. So if you don't mind buying a used one, you might check your local craigslist to see if someone is selling theirs. I use the BOSUs at the gym. Not every day, but once in a while I use one.
Do you need a BOSU to be fit ? Nope, but it is nice to have to change up your routine.
There are cheaper peices of equipment you could buy that would perform similar functions. A DYNA DISK is one. The other is a wobble board. It's the same idea. Standing on an unsteady surface helps to increase your balance which in turn helps to increase your core strength.