The book's great. A couple of things aren't very good (the class defense bonus has weird values, and the variant metamatic system where you pay with whole slots is just nonsense), but the rest is good (if not for every campaign.
Gestalt characters are good when you want to have really powerful characters (like Forgotten Realms played high-magic with gestalt Charakters), have very few players (but don't want players to have several different characters), or want a campaign with low magic (meaning very little magic items), but not low-power (they won't have their countless magical items to rely on, but they will be powerful nonetheless).