I had one for labour, it was really good. Part of what it does is encourage your brain to produce endorphins that make you feel a bit high and block out tne perception of pain. You need to put the machine in and leave it on low for ages and build up.
I have had one for years, do not find it does any good for me.
I have arthritis in both knees, they used to sound like bags of rice krispies when walking.
I was in the process of going for knee replacement.
Friends of mine got me onto a cream called Uddermilk, seems more farmers use it for body pain than on the animals. I also take a tablet called Pycnogenol from Holland and Barrett, knees are like new. No crackling, no locking. No longer thinking of knee replacement and have put other folk onto this cream.
Boots do the Uddermint version for personal use made by the same people and had the very same ingredients around £8 for a small tube, I buy a 500g tube for £17.95 at the farm suppliers.
Good pyhsio, tens ok for short while, fine for muscle, but knees are far more complicated. Exercise to build muscle up and support it
Mine's not too bad at the moment,heat, and stretching