Since my games almost always take place at low levels and I like to use alternative magic systems, I rarely use creature stats as written. What interests me the most about monster books are the ideas for creatures, the specific mechanical traits are of much lesser importance.
And while looking up stats for a creature you know is very convenient, just looking at stat blocks really isn't helping with finding new creatures to enhance the world of your campaign. The picture of a creature and a few sentences on what the creator had in mind are actually the most important parts of monster books.
Unfortunately, what I consider an interesting creature is quite different from what the writers at Paizo and WotC have in mind. I was able to get a look at each page in the Bestiary 4, and there really wasn't a single entry which I would have liked to adapt for my campaign. The Grendel picture was cool, but I think it was a CR 20/ML 5 creature that would be impossible to directly convert to CR 8 or something like that. Bestiary 3 has the kami, which I really love a lot, but I think that's pretty much it. The D&D MM4 and MM5 were just as bad.
When looking for new monsters, I mostly hunt for really old monster books. Sometimes there a cool creatures I've never heard of before, and quite often I get a more detailed description of the original idea that was just glossed over in later editions, and which makes the creature much more interesting.