I have very problematic knees and regularly ice them after a gym session. In the past this has involved tying something around my leg to hold a standard ice pack in place - if I moved it usually fell off and I had to replace it.
These are well worth the investment - strap it on and forget about it for 10 mins.
I was worried at first they wouldn't be large enough (the icepacks), as they look quite small - but they hold enough chill to work for around 20 mins and the strap means you can apply a lot of pressure. My knees have certainly noticed the difference.