This? is wonderful. You meld the worlds of Harry Potter and Sherlock Holmes seamlessly, create a terrific crowd of OC wizards and witches and Muggles interacting with the characters we know (who seem heart-deep true to themselves in this setting, though both Harry and Draco have grown up very fast from the young men they were an hour before), a worrying fourth Hallow (it wouldn't be a Hallow if it didn't have the capacity to doom civilisation/all life on earth in the wrong hands), and a neat and complex plot which creates, uses and then resolves all manner of contradictions. (Phew. /superlong sentence.)
Watson as narrator: perfect. Snape and Holmes as collaborators: fascinating. Harry and Draco as young, powerful wizards growing up, getting along (with some stumbles on the way which the adults won't tolerate), and learning empathy for each other and for their "natural" enemies as well as allies: delightful. Snape and Harry as people in love and gradually coming to admit it as well as to act on it: endearing. Lack of pr0n: not a problem!: I can use my imagination if I need to; you've given me something I couldn't imagine.
I've spent hours reading this, and enjoyed them thoroughly. I don't doubt I'll be re-reading this story at regular intervals; it's joined my special collection of impressive historical/literary crossover AUs.