It seems like there's been a huge surge in people getting tumblrs lately. In the forum I frequent, there was a time where there were one or two statuses each day that announced they finally got an account.
I never rejected the idea outright, but I never bothered to get an account because, like, what do you do on tumblr? Seriously, it's this huge thing that I honestly don't know what it's for, because it seems like you can do everything tumblr does on, say, Facebook. Or deviantART.
I got one because my girlfriend at the time talked me into it, but I don't update it much. It can be used as an artblog if you want, but mostly people just reblog a bunch of ready made posts that are pretty ridiculous. If there's ever a definition of a hive mind, Tumblr is the perfect example.