I’ve had Synvisc injections in both knees. I just finished the third set of injections; you will feel some pain during the injection into the knee. The cold numbing spry helps when then injection starts, however you feel a slight amount of pain from the pressure of the fluid passing into the knee, the pain goes away pretty fast. If you need fluid removed from the knee this is unpleasant. Extra fluid in the knee makes it hard to bend and causes additional pain. I had fluid removed from my right knee and that alone made the knee fell better. I was told it would take a few weeks after my final injection before I would notice a difference. I can tell you that my knees fell better, the amount of popping and cracking with sharp pain are all most gone. Synvisc only works on about 60% of the people, I happy to say so far I am in that 60 %.
I haven't had Synvisc, but I've heard that the injections are quite unpleasant. The relief you're likely to get, though, is worth it. My father in law had these done and it helped him put off knee replacement for a couple more years.