Nice idea - but like a lot of the plugins I get from WPMU, not well executed. Here's some quick feedback I hope you find helpful:
1) Install worked fine on a 3.2 single WP site (not MU).
2) Subscribe form looks ok (not great) but does not return confirmation in IE8 or Chrome. Looks like others have seen this issue (jquery).
3) I don't see an option to use a shortcode in a page instead of a widget to show the form
4) Would be nice to be able to limit the capability to registered blog users. I guess I could edit the widget to do a check if user is logged in...
5) Unsubscribe sends a very transparent subscriber ID... any curious (malicious) user could simply iterate through IDs and unsubscribe people. Random token per subscriber would be safer.
6) Easy to subscribe anyone's email (no confirmation). Suggestion: when subscription is created, create the record w/the unique random token. Just like opt out, require a confirmation click from the user w/that token to "activate" the subscription. This option could be toggled on/off in plugin settings. Just like the subscription email, also put a field to customize the text in this confirmation email.
These are my quick thoughts. It is functionality we would love to have but not quite featured or reliable enough yet in this plugin.