In the game I'm currently playing, between my character with Elusive Target and the druid's dire wolf companion, we use the trip rules nearly every fight.
In my various Living Campaign experience, it usually comes up a couple of times every mod--more at low levels than at high levels. (My high level character has Improved Trip but he tends to fight a lot of things like advanced half-fiend legendary krakens, mountain trolls with fighter levels, dread wraiths, greater shadows, and icetouched white dragons. When he gets a shot at a kobold, orc, or a Yuan Ti, he'll use it, otherwise he'll cast an empowered vitriolic sphere or an empowered fires of purity and hurt his enemies.