People buy incomplete (in-progress) books all the time on Leanpub! Our customers are typically familiar with the Lean Publishing process, and if a book looks interesting to them, they'll buy it even if it is not finished yet.
There is actually no general way to rate the reaction to publishing a book, on Leanpub or any other platform. The reaction depends on how many followers the author has on social media online, whether they've done any marketing or posted about it on their blog, luck, timing, etc.
Our main recommendation is for authors to not try to compare themselves to anyone else, and to just consistently draw attention to their book online, with good blog posts, interesting posts on social media, using coupons to give free copies to potential reviewers or influencers, and things like that.
Here is an article from our Help Center that may be useful:
http://help.leanpub.com/en/articles/1587118-tips-for-self-publishing-and-selling-books-using-leanpub
Please note that for questions like this, the best place to ask them is on our Authors Forum:
If you post questions there, instead of just hearing about this from the Leanpub team's perspective, you may also hear back from anyone in the Leanpub author community, about their experiences and expectations and advice, etc.