Well, I made it to the final boss of the DLC, who I believe is generally considered the hardest boss in the game (and given how higher-tuned the DLC has been, I'd imagine that I'm going to have an easier time than typical for the last three main-game bosses I have.)
I have only the Rakuyo to retrieve at this point - I had a decent shot at it using the Shaman Bone Blade to charm one of the shark giants, but it hadn't occurred to me that that would wear off eventually. Not sure if this gets broken if you do damage to the enemy (I'd guess so,) but I'll probably try again now that I've restocked on the item (need to check the Messenger Shop to see if one of my badges lets me buy it.)
Anyway, beyond that I think I might have every last item out of the DLC. I certainly have the Blood Rock, though still no idea of which weapon to use it on. I've gotten a ton of armor sets and unusual gems.
I recommend the DLC if you're on the fence about getting it. I'd say it's definitely harder than the main game. Partially that's just tuning - they're expecting you to go in with a heavily upgraded weapon and a lot of levels under your belt. But the fights also involve some very fast-moving enemies. I'd say that Living Failures is the only "easy boss." Laurence is mid-range, and Maria's arguably upper-mid-range. Ludwig and Orphan of Kos are definitely in contention for toughest boss in the game.
For the hell of it, here's a ranking of bosses by difficulty in the main game, from easiest to hardest. This is totally subjective and maybe skewed by flukes like my one-shot of the Shadows of Yharnam.
1. Witch of Hemwick:
If you know what you're doing, this fight could not be simpler. Just don't get in front of her and make sure to split your damage between the two. Just run from the stalkers if you have any up.
There are scary parts of this fight, and it's not immediately obvious that its arms are the weak spots. Be sure to run away when the eyes come out and you get lasers.
3. Shadows of Yharnam:
This might be a lot higher on the list, but I figure if I one-shot it, I have to put it before any that I died to. Summoning an NPC helps immeasurably. Kill Katana guy first, then the caster who shoots multiple fireballs, and then the leftover guy.
4. Cleric Beast:
Appropriately easy for the first boss (sort of) of the game. Just stay behind him and if you're having trouble, use fire!
5. Celestial Emissary:
I honestly forgot about this guy. If you can figure out the trick and avoid the little guys, he goes down relatively fast.
6. Father Gascoigne:
Definitely a wake-up call boss, but if you get good at parries he's not too bad.
7. Blood Starved Beast:
A "no-really-wake-up" call boss, BSB is actually not too bad when you figure out how to dodge it and the threat of the poison is more psychological than real.
8. Rom, the Vacuous Spider:
This is a dance boss. I know there's an "ignore the spiders" method, but the best way it worked for me was to stay un-targeted so I could see what Rom was doing at all times and just take out the spiders until they were gone and then get some nice time to focus the boss.
9. Vicar Amelia:
A serious challenge, and much like Rom, an appropriately daunting gateway boss. I suspect I might have a significantly easier time on my second character, but Amelia is definitely punishing and tricky.
10. Ebrietas, Daughter of the Cosmos:
Ok, I actually had a much easier time on this one, but that's because I'm leveled up far more than I think I'd be expected to, given how I've been doing the DLC. I'd expect this fight is more on the aneurysm-inducing side if you're lower-level, with a nasty charge and some unpredictable tentacles and apparently some magic attacks that I never saw?
11: Micolash, Host of the Nightmare:
The main difficulty is not getting frustrated here. Phases one, two, and three are not too difficult, but they take a long time, and it's phase four where he's most likely to kill you, meaning you'll have to go through all that labyrinthine chase over and over.
Not Yet Fought:
Mergo's Wet Nurse, Gehrman, and the Moon Presence.
I'd expect to place Laurence at 8.5, Maria at 9.5, Living Failures at 5.5, Ludwig at 11.5 and Orphan of Kos at like 15 or something.