spiked chains are great, one handed or two handed. the real key is that you don't need to swap longspear for longsword... you can attack at range and in close combat. so when you focus and specialize, it really helps. now, you may not do as much damage as the fighter with a greatsword, infact, you'll average 2 damage less per hit. but, you can make up for it in the numbers of hits on AoO, the manuverability of hitting someone 10 feet away, and the ability to avoid AoO.
a psionic warrior with the "stand still" feat can empower his chain so that if someone charging at him provokes an AoO, he gets to swing. if he deals damage, they have to make a fortitude save(DC 10 or 15 + damage dealt by AoO) or be forced to stand still and not continue their charge. truly one of the best combinations in the game.
lastly, as mentioned, just because you cover so much ground, you demand respect. spellcasters can stand behind you, you can hold off monsters in a 25 foot wide corridor, and with cleave, you can take out quite a few opponents.
overall, the spiked chain doesn't do as much damage as a greatsword, but just compare the versatility of a fighter with a greatsword vs. a fighter with a spiked chain. one might be a better one on one opponent, but as a fighter, your goal much of the time is not to just deal damage, but to occupy opponents and take damage while archers, support fighters, and casters can have time to get in position or do their own deeds. a spiked chain allows you a number of combat options that you don't have with a greatsword.
isn't it annoying when you run into 5 opponents and the fighter charges at a couple of them, leaving the others free to assult the rest of the group? with a spiked chain, your always within a 5 foot step and a 10 foot reach of smacking that guy trying to hurt your wizard friend.
