I own Tome of Beasts, Tome of Beasts II, Creature Codex, and Deep Magic.
The monster books are really good. Art and design is great and the monsters are much more varied in abilities, rather than just being different sacks of hit points and damage with minor flavor differences, which I feel most of the monsters from Monster Manual and the other WotC monster books are. But what I like the most is that the monsters are more challenging than the official books, these monsters actually challenge the players. And then there are all the pawns for the books
Deep Magic, however, didn't impress me. The spells don't feel balanced (often weak compared to spell level) and some feel downright pointless. The wording in a lot of them is a bit off the mark.
There is an 8th-level spell that causes creatures with darkvision to be blinded and stunned for 1 round, creatures without darkvision can save vs. Con or be blinded. It affects all creatures that can see the caster, even if they're wearing a blindfold...
There's also a weaker version of Feign Death called Heartstop, which I don't really see the point in having, as Feign Death is only a 3rd-level spell and can be cast as a ritual on top, which the other spell can't.
But still, their monster books are incredible.