I vote "not worried" about spiked chain and Improved trip. If a player intends to use it cheesily, then make em aware that all somewhat-intelligent combatants will instinctively try to avoid AoO's and thus stop at the reaches edge - and note that such stopped combatants also provide soft cover, meaning that the next combatant can usually walk a field further without provoking an AoO.
Seriously, It's annoying maybe, but not horribly so, and not much more so than many other player abilities. The druid with the high wisdom casting plain old entangle tends to stop my humanoid masses far more effectively, and that's just a level one spell. And entangled creatures take a -2 attack and -4 dex penalty, meaning that they can't even use ranged combat very effectively. A spiked chain also doesn't work well when partially surrounded by allies.
Also, enemies can (and do IMC) tumble to avoid AoO's, so if they're the dextrous type, let em try that. Failure to tumble just means they take the AoO nevertheless, so it's not really a problem. Finally, you can also oppose using Dex, so some combatants actually manage, despite being human. And even a strong fighter (20 str, +9 trip check) tripping a halfling (8str, -5 trip check) has an unfortunately high chance of actually being countertripped, which actually therefore occaisionally happens (though more usually with stronger opponents) and is very annoying to the fighter. And unlike many other specialists, this one can't really scale well: he'll never be much good at tripping ogres, and be outright useless against larger opponents.
Having seen an effective, but not game-breaking exotic weapon master maxed out tripper, I'm not worried. It's more tactical work for the DM, but not problematic.
