We send mass emails - but we keep the details short and sweet (what's going on, what date, which servers, what will be affected, how long will it be affected) and link back to a forum post with more details. That way, if people want to know full details, they can follow the link and read the whole story - and if all they want is a quick notification of expected downtime or a notice that something's currently in progress, they can just read the email and get the gist.
We try and summarize any major upgrades in the quarterly newsletter. Keep it to one sentence, link folks to the forum where all the announcements reside, et voila - they get the summary, and can go read the full details on their schedule.