I'd gotten Raid and Ruins, and enjoyed both of them. So, I ordered Black Dog. I don't regret the order, but it's not of the same caliber as the first two. It's more techno-sounding, less "spooky" and many of the songs seem, well, repetitive. The same sounds, arranged ever-so-slightly differently, with a new name.
From what I can tell, the music is only off the first 1, maybe 2 OVA's. I was hoping for a bigger selection, and my fingers are crossed that when they finish the series, they'll release a full set of soundtracks. It's not bad, it's good background music, and it's certainly intricate and very different. But it's just "okay." I like it, I'd get it again, but it's just not anything fantastic.
Worth getting if you're a Hellsing fan. Otherwise, you might want to pass on this CD, or wait for the next soundtrack to come out.