I was able to make it down to bloomingtons tournament last weekend. I brought my space marines this time and decided to give them a try. I had gotten a practice game against a Knight primary army the day before with my tau, and it seemed I was going to get practice in with my marines now too.
My first game was against a Knight list, with space marine allies. It was a five objective missions, so I though I had the advantage. We had night fight and he deployed spread out, so I refused flank to deny him good shooting first turn.
First turn went in my favor as I killed his sternguard, his droppod, and his first knight. Second turn my storm talon blew up his storm raven. From there it went down hill as I got poor assault distance rolls, he got remove from play stomp effects, and I failed about three ld tests in a row while forgetting about atsknf (I'm a xenos player.....shhh)
The game ended with him having one knight left, and I had a tac squad on the other far side of the table for a tie. I won by linebreaker and first blood as we both had slay the warlord.
Overall knights don't seem to be a push over list. They are beatable just like the deathstar lists, and can be a bit rock paper scissors in match ups. Some lists will struggle against them, and others won't. If your army can handle AV13 walkers, you should be fine.