I've spent the last hour or so messing around with GtkRadiant.
I would say that at least initially, it has the user-friendliness of ... um .. well it isn't user-friendly at all and quite non-obvious on how to do everything.
I'm also shocked by how poor the documentation is.
I suppose if you put a great amount of effort into learning it, it is a great editor that can do great things, but it has the user-friendliness and documentation friendliness of a hobgoblin.
The night is young. How else can I annoy the world before sunsrise?

Inquisitive minds want to know ! And if they don't -- well like that ever has stopped me before ..