I think trading down and going for Justice or Ngata if he slips is the best option. However, taking Ngata at 8 would not be a bad pick. As I said in the Ourlads thread, I'm a huge Oregon fan and saw Haloti play pretty much every game he played at UO. First off, Haloti is huge, but he's not a Gilbert Brown that will eat himself out of the league. He pretty much stayed at about 340 to 350 his entire college career.
Second, he did tend to take plays off early in his career, but his last season he was much better about playing hard. I think people say he's lazy because he was often double and triple teamed so it was hard for him to get a consistent push on every play, which caused people to think he wasn't going 100%.
Finally, there shouldn't be any worries about his knee. His injury came in the 1st quarter of the 1st game of his Soph. season (on a punt no less) but he played his last 2 years without any problems at all. Not saying he can't get hurt again, but he seems to have recovered well from his last injury.
Just my two cents since I've seen him play quite a bit.