I was hoping someone would reply to your query too.
I do NOT have one, but have been seriously considering it. Keyboard Magazine gave the related 100 a very good review and said the sounds were as good as many more expensive keyboards and the feel was excellent.
I have tried one in a store. I'm not a seasoned player, but the board felt pretty good.
If I was a Windows user I would have bought one by now, but since I am a Mac user, some of the advantages of the 400 would be lost for me.
Another caveat is that Casio doesn't seem to be very supportive, having shut down their own user forums, it seems.