I had never used knives before H&H, and decided to test them out after that update, due to what the patch notes said about them. It was love at first swing, pretty much. I had an absolute blast wrecking boars and greylings (I started a new character) without them even getting a chance to take a single swing at me.

The secondary attack is super fun to use, and does a ton of damage. The sneak bonus is awesome, and the fast attack speed remains a huge bonus throughout the game.

The Abyssal Razor is possibly my favourite weapon in the game, of the ones I've seen (= all but blackmetal ones) although it does eventually get outclassed by the silver knife.
It is even possible to obtain the Razor very early on -- I am about to get it once I get the first pick, as there is a Leviathan almost within swimming distance so a raft is enough -- allowing one to skip the copper knife (which means less copper needed).