Are you referring to free upgrades or paid?
From my experience, the free upgrades for elites are few and far between. I usually ask at the gate what the situation is (in terms of empty Biz seats and how many other people are waiting for free upgrades).
In the end, if there are seats available, I usually pay the $35 and bump someone off the free elite upgrade list. For peak travel times (Thu/Fri/Sun evenings), they are most likely sold out in advance.