I thought I'd try the balance ball exercise thing - it looked fun, didn't cost much, and would shake up the exercise routine - but in practice, it has turned into 101 Ways To Bugger Bad Knees.
One of the crunch exercises (put knees on ball, balance on outstretched arms face down, pull yourself into a downward dog position rolling the ball along your shins) really did do wonders for my abs, but it did horrors for my knees, to the point that I limped for two days. So I decided that I'd take the ball in and sit on it instead of a chair at work.
Would you believe that THAT has crapped my knees too? None of this "shift your core to stay balanced" thing, as I rolled around I used my legs as braces and brakes, meaning that both knees were bearing weight while bent. And that's the one thing that my knees simply cannot do.
Feh.