Hazel + May = Crowded (sort of)
Owl, I too had wanted to get over to Hazel to say I have done it, but honestly I was amazed at how many other folks were over there when Hans and I made the trip last year. Now don't get me wrong, we're not talking about tailwater type crowding. You should be able to find lots of free water, but it doesn't have the same "hidden gem" feeling that many of the other places in the park, that are off the beaten path have.
On weekends in May you can expect that the campsites will be full (or close to it). If you can go during the week, you would be much better off. But even if the campsites are full, its a pretty big stream; you should be able to find some water...
The trail is easy, but if you have a bum knee, I would heed Hans' advice and stick to 85. There are a couple of sites right along the creek, but I think the ones on the other side of the trail are nicer and more secluded. This way you can drop your pack and head up stream (or up trail) to your heart's content.
"Even a fish wouldn't get into trouble if he kept his mouth shut."