Hi Cafe.
I have thought that a more interesting metric might be to send the maintainer an email when their package stops building automatically on hackage.
I think, this is "must have" feature for new hackage. If error was occured during build, send email to maintainer: "Error occured while building your package <name> with GHC <version>. Build log: <...>. If you are to fix it in a week [month?], please send reply for this message". If answer was received, then do not send such notifications every time during specified period (say, a week or month). If after end of this period, package still could not be build, then send next notification, maybe with additional question: "If you are to maintain this package, please send a reply for this message"... With any scenario, "build failed" notifications is a "must have" feature. WBR, Ilya Portnov.