Other than perhaps individual/personal opinion getting in the way... I have no clue why people have given this title such low ratings.
Tome of Secrets was the first 3PP product I bought for Pathfinder (in late 09, early 2010) and I must say that it's still one of my go-to's for ideas.
Tome of Secrets covers a lot of ground. From ways to modify your favorite monsters, unique races (Saurian have become one of my favorites), Tinker Mages (Artificer) and the best Swashbuckler I've seen for 3.5... to interesting flaws and awesome chase rules. You or your party can take this book to enhance roleplaying (instead of just roll-playing, like many supplements).
Not to mention they will give you a free PDF even if you bought the book from a local store! Now that's service!
I would give Tome of Secrets 4 stars, but I feel that many of the reviews here are very unfair. Especially the 2 and one star ratings that people left NO INFORMATIVE CRITICISM for.
Gamers! Give Tome of Secrets a shot. It's definitely worth it.
Paizo, please crack down on this stuff. Objective and Unbiased versus Ignorant and Careless.
Although I liked the base classes and some of the content, there is a lot in need of fixing. I can stand typos and such, but I really feel too much of this is still stuck in 3.5 edition. I was most excited about the million item list (3 d100 lists) as the Diablo II: Awakening book from 2nd ed had this, and I liked it there, but there really isn't 1 million items. It felt a little ripped out of the diablo list without much change (which I kinda liked), but in the prefixes alone are missing entries 18 and 42, and the +1 to attributes are repeated, making the list feel a little underdone. I don't care that this list is unbalanced, but I got annoyed having to reroll this. I'm giving it 2.5 stars because I happened to get it on sale really cheap, which made the price an okay trade-off. This book came out around the time the final rules did and was one of the first third party to give something substantial. I just caan't understand with all these errors and reviews pointing it out why there iesn't an updated file.
Lots of good ideas, not quite as many good executions . . .
I have to admit, I was more interested in the class concepts in this book than I have been with some of the other 3rd party books I've read for Pathfinder, but the execution on many of the classes, including some of the potentially interesting classes, just falls short, to the point of not really being able to figure out how to implement them.
Overall, its not a bad buy to pick up and sort through what might work for you, but be aware that, while it seems like there are a ton of good ideas, there are also lots of places where a good editorial pass would have fixed some problems.