I do not have it but this is essentially what it offers: It will entitle you to quicker repair services when needed and the ability to go online and make advanced Genius Bar reservations (and choose the Genius you prefer). Also, certain workshops become available plus discounted prices on both Apple and third-party products.
If those things are especially meaningful to you, it could be worth the $99 a year fee. Personally, I think it is one of those premium Apple offerings that is excessive in price and not worth it. However, your mileage may vary.