I've used a Belgian Blue (haven't had the pleasure of those coticules the straight-razor people use and love). For people who know what they're doing, they're probably the bee's knees. For me, it was a slow-cutting ordeal with purple slurry (kind of like the one Cookie Crisp makes in milk) as its only saving grace.
The pal who sent it to me not only told me to keep it, but didn't put a return address on the box he used for shipping. I paid it forward to someone else and he wasn't happy about it, either. While all three of us can do some good sharpening (the other two much better than me), it's very fair to say we weren't people who knew how to release the Belgian Blue's magic. Others' results vary greatly. Also, as a natural stone, some can be awesome and some can be wretched - just look at the dozens of types of aoto stones...