I've browsed their wares a number of times prior, but they seem to carry a smaller range of diamonds and they tend to have more modified colours or slightly cheaper (as far as FCDs go) inventory.
Occasionally I see something exciting though, but Langermann seems to have a better range of delicious colours.
Never dealt with them though, not even through email.
ETA: Arrrgh, the gorgeous sea blue gem they have that I went back to look at is gone!!! Sigh. I need more PSers to buy these things before they disappear into vaults.