It's reportedly very painful (assuming kneecap and not "dislocated knee" as above; my former roommate, who played college football, had a dislocated kneecap and said it was excruciating, in part due to the muscle spasms he had while his knee was dislocated), but depending on the extent of damage to surrounding bone, cartilage, or ligaments, a dislocated kneecap is potentially less severe and less of a recovery than an ACL tear.