I tried logging in and commenting here on an iphone (safari) and found it doesn't work -- the form gets all shaky and won't open all the way.

Loss of control over the comment form and subscription emails is what bothers me most about JetPack Comments. You can't style the form, and every WP.com/JetPack blog/comment subscription gets the same blue template as every one else using it. If you often get several of these from different sources the lack of differentiation is confusing. There is also some embedded intro text you can't change; this includes use of the term "blog" which should not be assumed to be appropriate in all cases.