I'm a subeta regular, but haven't checked the forums all that much. How much do people use them on a daily basis?

For the most part no, but it seems like there are exceptions. The general off-topic chit chat forums, and the user run games, and the custom wearable forums, all seem to have regular activity. When someone hosts a user-run event, like the old Style File weekly avatar contest, or the current pet contest spotlight hosted by the Menagerie guild, those get a boost in activity.

The community is really active, but the threads might not be very fast depending on the topic. There are a lot of social groups be it Custom Wearable communities, Forum Groups, Chit Chat groups, etc. that might have private forums or group threads that they post on. Many of these groups/communities welcome new members with open arms :)

If you are after a more fast-paced chat, you can activate Shout Box under Personal > Preferences > Widgets