What's the deal with the Balance Cushion?

If you belong to a gym,chances are you've seen one of these contraptions kicking around. What are they, You ask? They're balance cushion -- small inflatable disks that you can use during your workout to work on your balance and stability. It ultimately benefits your core and can add a bit of challenge to a routine.

Still can't picture it? Fitsugar recommends a number of ways to use one, including:

• Standing on it while doing squats or lunges
• Placing it under your back while doing sit ups
• Use them under your feet or hands when doing push-ups or plank.

I've never used one but I'm curious to know what it's like. Any feedback?