Can't Speak For The Previous Poster's Experience..
But I clear my upgrades around 80% of the time as a silver. Even travelling tomorrow (probably a fairly busy travel day), I have cleared my upgrades 24 hours in advance.
Even if I don't get the upgrade, I like boarding first... I like being able to select premium seats like 5F (almost first class... for my feet at least).. I like being able to use the Silver Elite line and not having to wait 38 minutes to connect to a phone agent when I need assistance.
I think it's well worth it.