[Review edited upon further investiagation; I'd now give this 4 of 5 stars]
The importer portion of the plughin seemed to work generally fine, but it appeared to wreak havoc on my Tumblr account. Initially I thought it had deleted and heavily modified my Tumblr posts, but it turns out that they are all rewritten and then reposted to Tumblr with modified URL's to point at the imported WP posts. I'm sure this didn't do any favors to anyone subscribed to my RSS feeds.
Further, it would have been far better had the posts been imported into WP as "drafts" or "pending review" and allowed me to publish them as desired. Instead they import as "published" which wouldn't have been so bad if some of the Jetpack social media integration didn't auto-post the hundreds of them in bulk to four different social media platforms. I ended up spending more time cleaning out the social media cruft that vomited into Twitter, Facebook, G+, etc. than it would have taken to cut and paste all the content over by hand. The expected behavior should be better delineated so this social media fail could have been avoided.
Those using JetPack, IFTTT, or other social media plugins should DEFINITELY turn them off prior to using this. I can't imagine the problems that might occur if you've got an auto-post from WP to Tumblr turned on when using this.