Thanks to gameblog reader and Hackmaster superfan Topher I've been able to recently acquire the entire 8-volume 1,600-critter Hacklopedia of Beasts. Thanks again, Topher! I've had a great time flipping around in these books and reading various monster entries.
If you run a first or second edition Advanced game you might want to check these things out. Monster statblock compatibility with those games is very high. I'm not saying that every DM needs to own the entire set, but you could spice up a campaign nicely with just one or two volumes.
Back when I was running 3.x I owned and used fairly few monster books. Sure, several books had a monster section that came in handy. The Epic Level Handbook's critter selection was very useful when my Wild Times campaign started inching its way up and past 20th level. But in terms of pure bestiaries I only owned maybe four total and none of them, besides the Monster Manual, were from Wizards. And upon reading the Hacklopedia I think I finally figured out why I never got the new Fiend Folio, or the later Monster Manuals.
Every once in a while I would go into a bookstore or nerd shop and check out the WotC critter books. Sometimes I even had opportunity to pick up one cheap, like the time I found the FF in the discount bin of my favorite local bookstore. The problem has always been that I flip through the book and I never see anything fun. The monsters in WotC monster books might be fun when the dice hit the table, but there's nothing in the book that shows me that upon a casual perusal.
I can open up a random Hacklopedia and see a Map Snatcher running off with some poor party's dungeon map or a qullan sheering some fool's skullcap off with his preternaturally sharp broadsword. I can read about how sirynes get annoyed that men seek them out for their looks rather than their singing. The Hacklopedia has all the loopiness and sheer joy that I get out of original Fiend Folio or the monster sections of the Arduin Grimoire. I get the distinct impression that someone had a heckuva a lot of fun writing up these monsters. With WotC products all I see is grody art and monsters designed mostly by mechanical concerns. "Hmmm, we need an aberration type that is aquatic but suitable for CR 2." That sort of paint-by-numbers thing.
Again, this is only my opinion and only based on surface impressions.