Well, so far, I'm not too sure that I'm going to find much in Blackmoor. Despite the attempted tie-ins to Known World in the past, it seems that original setting that Arneson created is a tad too off-the-wall for me. Plus it seems that quite a bit of what has been published about Greyhawk has actually been an amalgam of Gygax's Greyhawk and Arneson's Blackmoor. I have to wonder, thematically, what does Blackmoor have to offer that's any different?
Bingo.

Never seemed to stop Greyhawk, though, eh?
Well, apart from telling your players that they're playing in something approaching the first fantasy campaign setting, not much. FFC Blackmoor feels like a wacky homebrew, not that there's anything wrong with that. If you like nostalgia, or the idea that you're playing in a vintage setting, I guess that's where Blackmoor's appeal lies. Mind you, the setting got a tune up, facelift and tummy tuck when it was added to Mystara, and most of the cheesy wackiness was shown the door. It may well become better, faster, stronger this time around too.
Again, the oldskool nostalgia factor seems to work for Greyhawk...but then again, Blackmoor wasn't really "around" as much as Greyhawk was, so despite being it's senior, there's less of the "I played this when I was thirteen" factor to tap. It'll have to rely on the vintage D&D* thing instead...but in theory, nothing beats it in that department except maybe the Castles & Crusades Society map...
At the end of the day, it's worth buying just to out-oldschool the Greyhawk enthusiasts.
*: Not that it really was a D&D setting to begin with, relying on Arneson's own rules during play from what I gather, which in turn helped "inspire" D&D...