First let me say that my clients are extremely pleased to discover this plugin. Good stuff!
There are a few small (non-critical) problems i have encountered with the subscription sign-up widget.
1. The default option of label in the box does not work (Firefox), and it's not very obvious that to get the name label you have to change this option, save, then set the label. A detail.
2. We are getting messy PHP error messages displaying in the widget area when the email input is invalid (i.e. empty). I had to hide some elements and filter the error text to elimiinate this.
3. When the above happens (as well as success messages) something is corrupting post data so that the title for the current page obtained with the_title() has gone blank. I've worked around this by using single_page_title when the normal title fails to deliver text.
4. The success message is editable, but the error text is not. That is important to us because we are using the widget in more than one language on the same site. We would like to set the invalid email (and any other) error messages for the target language, but cannot.
Otherwise, so far, a very impressive plugin. Even has SMTP option for outgoing email (which we needed). Thank you.