I been using it daily since forever, so your definition of ready for daily use in that context imo is just fine.
iirc its going beta anyday now, since the skins abi has been bumped ready for beta yesterday, so skins wise you may find that from yesterday it likely will break most of them, since up to now the aby was 4.00 and now its 5.0.0, with a bit of luck skin devs would have found 3 months to get ready for this if not just ignore the skin xxxx has been marked as broken.
Yes newer xbmc version will upgrade the databases version, so if you want to keep in sync all your instances of xbmc need to be same. Otherwise you can run one machine on Gotham but not synced to others.
Since I only use development master branch I dont care for 3rd party skins, they are really the cause for many instabilities and problems in xbmc, so I only use Default skin and any addons that I need want.
You should always backup
userdata (wiki) and no it shouldn't need to reimport anything or rescan anything unless you remove
userdata (wiki) after backup.
There is a issue with a likely PR to fix on internal forced subtitles. Yes Subtitles providers (Opensubttles, subscene etc) is now separate and for each provider you need a new addon Opensubs is default I think.