This is a very solid expansion to Bloodborne's amazing foundation.
I think the levels in this could be a bit better. The opening area had a couple awesome visual moments, but I don't know... felt a bit uninteresting, had some recycling of ideas from the main game, and an un-fun long-ish run back to its tough optional boss. The second major area was cooler, but full of annoying enemies and didn't really utilize its shifting layout to the full potential. The final area though is kinda short but really awesome-looking, the freshest thing in the DLC and very memorable.
Boss-wise, I can draw a pretty clear line. I think the two standard humanoid bosses have such awesome movesets. Their design fits so brilliantly into Bloodborne's combat, demanding mastery of at least a couple of its key mechanics. The two giant beast bosses were not my favorite, though. The first one I can at least say was exciting and well-presented. The optional later one I really do not like. The camera in this fight is a mess, his damage output is the most obscene in the whole game, and his moveset and AOE damage is so aggravating. The best thing I can say is that he forces you to play differently, but I struggled with him the most by far and did not have an amazing time.
I'll use my favorite boss, the penultimate one against fellow hunter Lady Maria to demonstrate how I think the balance can be improved. A large amount of her difficulty comes from an ability to instantly kill you with just one or two moves. The result is that for most players I'd guess, her fight hinges on whether you have enough health to tank those hits. I think instead to provide a more consistent and less frustrating experience across the whole playerbase, they should tone down that kind of damage, but up her evasiveness and resistance to stagger. Then she is arguably more interesting and challenging to fight, especially for high HP characters, and less punishing for low HP ones.
Accessing the DLC is a bit cryptic and fits awkwardly into the main game. In my opinion, this kind of content would benefit from being more clearly accessible at pretty much any time, with some way for you to get an appropriately-upgraded character for it, at least if you've already cleared the main game.
With that being said, I do recommend including this DLC in your first or second playthrough of the game. It does add some pretty fun and memorable moments, and if you like, you can tackle it with an upgraded weapon and high stats right before the base game ends to smooth out some of its more punishing edges.