3-Of limited use. Unless your DM uses lots of Grease spells and most of his monsters have Improved Trip, you won't get much use out of this one at all. In the various campaigns I've played (including over 50 Living Greyhawk modules and probably a dozen Living Arcanis modules which gives me reason to believe that this is a general D&D thing rather than a peculiarity of the particular DMs I've played with), I've only seen villains use trip attacks in two situations--one was a module I wrote that gave the villains improved trip and I was DMing for the other.
More importantly, this doesn't remove the +4 to hit you because you're prone so it does nothing to mitigate the primary advantage (at least at low levels) of a foe improved-tripping you. Compare this to Close Quarters Fighting which gives you an AoO (enabling you to inflict more damage on your foe) and enables you to add the damage to your grapple check (usually giving you a very good chance of avoiding the grapple altogether) or blindfight which enables you to retain your dex bonus and reroll any misses due to concealment. Both of those mitigate or eliminate the primary advantage of the foe's ability. Prone Attack doesn't.
It also suffers from lightning reflexes as a prerequisite. This is a feat which is, IMO less than optimal for fighters (except maybe dex based archer or finesse types) who will rarely make reflex saves anyway--even with the +2--and generally have enough hit points to be able to survive the effects of a failed reflex save.
In short, in comparison to other defensive feats like Close Quarters fighting or Blindfight, its prerequisites are less useful, in most campaigns, it will come up in fewer situations, and in those situations, it is less useful.