Some very good ideas here, and some of these character classes can actually be used as-is. Most books of alternate/prestige classes are badly unbalanced, but if anything these sorcerers are slightly lower than average with very strong abilities in one or two areas. Specialists are slightly harder to GM but have a lot of storytelling potential.
The best of the lot are the Numeromantic Sorcerer, who starts out mostly as a depowered version of the class but rapidly gains some fascinating abilities; the Immanent Heresiarch, a worshipper of human potential who seeks to destroy the gods; and the Follower of the Sixfold Septateuch, a millennarian cultist with powers over mathematical dimensions.
These classes are pretty arcane, and possibly suited more for an "intellectual fantasy" campaign (like Planescape) than rough-and-tumble sword-and-sorcery.
The extras aren't as impressive as the main text. There are two short stories leading the book which aren't bad but whose presence doesn't make much sense. The magic items are another set of Baubles and Urus, additional varieties of the same magic item type in most Unorthodox books. The selection of spells is strange -- many of them are very unimpressive, and one of them amounts to a date-rape spell.
Extras or not, though, the main thrust of the book is its character classes, and those are useful and have great campaign potential.
[5 of 5 Stars!]