Interested in what actual user sentiment is towards sponsored posts on Tumblr? Take a look at posts tagged with "sponsored posts" on Tumblr.
While there are a lot of negative comments out there, you do have to take it with a grain of salt. Any platform that's being adjusted to let advertising in on the conversation is bound to feel some growing pains.
I believe that Tumblr is in a critical place right now where they're really going to have to cultivate and work with the brands sponsoring posts so that the posts stay true to the platform and relevant to the audience.