Having tried Calligra for a few days, I can’t say that I’m a fan. The UI is pretty bad and it lacks a lot of functionality when compared to LibreOffice.
Development seems to be going very slowly. The release notes for the latest stable version, which supposedly represents two years of development work, note only a couple of changes for Calligra Words (probably the most used app in the suite). Compared to the massive development we see in LibreOffice, I don’t see the gap in functionality closing any time soon.
Also of note: Kubuntu chooses to ship LibreOffice, even though Calligra is supposed to be the office suite for KDE.
Is it really that bad to keep GTK around? Most users will end up installing some GTK app at some point anyway. At least if it’s included by default, themes can be set up to make everything look cohesively “Lubuntu-y” instead of having the first GTK app users install look horribly out of place.