I never did much with it - I'm working longer hours than I have in the past, and have a longer commute - so I need to find my fandom interests with pinpoint accuracy and not waste my time in something that chaotic.
Awhile back at Escapade convention, I attended a "how to Tumblr" panel. The answer to one of my questions surprised me. I explained that I'd been accessing Tumblr via Google. I'd go to Google, type in a fandom-specific meta question (such as "historical context for Steve Rogers"), and frequently Tumblr posts would pop up. I found a lot of great Tumblr posts that way.
My question at the panel was, here's what I'm doing - now, how do I find this same level and quality of information directly on Tumblr?
The answer was: stick with Googling.
I won't even talk about the lack of ability to have real conversations with real people there. I know people could and did form communities there, but I always found it an impenetrable wall, and due to my schedule I could never devote enough time to learning all the tweaks and add-ons people recommended.
Posted to LJ and Dreamwidth. Comment wherever you prefer. :-)