When looking at the human evidence in particular, nigella sativa appears to have a general health protective effect of a very small magnitude. It is able to reduce triglycerides and glucose when elevated and can normalize cholesterol metabolism (although not HDL-C) as well, but the amount of benefit it confers is too small for it to be considered a good intervention. As a general antiinflammatory agent (such as its usage in rheumatoid arthritis) it also seems modestly effective.
It does appear to have more promise as an immune booster (no evidence on rates of sickness, but it appears to increase the ability of macrophages to attack invaders) and an anti-allergic compound (with efficacy against allergic rhinitus, asthma, and eczema). One study that used nigella sativa alongside a proton pump inhibitor noted that the seed extract was potently anti-bacterial against heliobacter pylori with a comparable potency to the reference drugs used.
Although much more evidence is needed on this last claim, in men with central obesity the seeds have been noted to greatly improve subjective symptoms of obesity. Symptoms such as loss of libido (affected 84% of men), forgetfulness (89%), high appetite (73%), aches and arthalgia (74%), sleeplessness (54%), and laziness (74%) amongst others were noted to be outright abolished despite no changes in placebo.