Lilac fragrance is wonderfully intoxicating however it is very difficult to capture the scent and only until recently is there even a true essential oil made from the flowers.
The flowers are edible and have some medicinal qualities. I have to say eating even a single flower raw is a flavor exploding experience with slight astringency (drying to tissues), almost bitter, and very floral. I would say these are best for garnishes and edible flower displays on pastries rather than whole meals.
Medicinal uses are still a gray area when it comes to just the flower. Most resources that I have found (random blogs, pfaf.org, A Modern Herbal) list that the medicinal benefits of Lilac come from the leaves and fruit. Apparently used as a tea or infusion historically it has been used as a anti-periodic. Anti-periodic basically means that it stops the recurrence of disease such as malaria. There has been some studies that indicate a febrifuge action that may help bring down fever.
Lilac flowers have astringent, aromatic, and perhaps a little bitter qualities. Astringents tighten, draw, and dry tissues such as skin. So a wonderful application would be a cold or warm infusion to use as a toner on the face. Or using the same method but apply to rashes, cuts, and other skin ailments.
An aromatic action causes irritation to the place that it is touching (think GI tract) and irritation brings blood flow and blood flow equals healing! Eating the flowers raw may help with gastric issues such as flatulence or constipation. Making an herbal infused oil may be a great way to capture the aromatics for healing purposes and to make your own fragrance oil.