I posted a thread with a similar question (I'm a bit too lazy to dig it up right now)... the consensus seemed to be that secondaries were somewhat important, but not something to be pored over like the AMCAS primary. Of greater importance was simply finishing them off quickly and turning them in.
I've come to a general rule of thumb: the longer and more tortuous the secondary, the greater weight it is given by that school in its admissions process.
Unless the adcoms are sadists AND masochists (which, hey, they may very well be), they're not going to be forcing people to write 6 essays and THEN scrutinizing all these essays... that's way too time consuming to do just for the hell of it. On the other hand, schools that only ask one or two nominal questions (like "explain any felonies if you've done 'em") are probably not giving it much weight - unless you've done felonies, of course.