Everytime there's a new Twilight coming out I think the interest is waining and it still does steady box office. The audience is simply there. The trailer views for BD2 broke records:
http://www.deadline.com/2012/06/twilight-saga-finale-already-breaks-record/
Also, Harry Potter and DH was split in 2 and DH1 actually had the least audience for any Potter film. Some people just don't go to the first halves of movies when they're split. The same could happen here. Whether I'm feeling the hype or not I've given up doubting the audience for Twilight movies. They seem very loyal to the series. I'm not saying it's going to make $300m but this is an action packed Twilight and the last male oriented Twilight film, Eclipse, did $300m and this one will have more action than that so my bets are on it making that as well even though it will not be a summer release.