Marco.org - The Tumblr Backup app hits on an important point and summarises it perfectly:
I think a telling mark of any platform or software product is not only how easy it is to get your data in but also how serious and committed are they to letting you get your data out. I have ignored this in the past and it has burned me practically every time (I’m looking at you Evernote).
I think everyone has experienced data lock in at some point or another – Google docs recent addition of an ‘export everything’ function automatically made me trust the service more and want to increase my usage.