The Spiked Chain is only slightly overpowered on a Medium creature if used and abused properly. Its true power shines when in the hands of a Large creature. Even then, the Spiked Chain can be circumvented fairly easily by a savvy DM. If a player starts getting cocky with Improved Trip, guess what? They just earned a trip to an evil Yuan-Ti monastery. Not much tripping against those snake-tailed Halfbloods, Abominations, and their huge snake minions. Or, simply send a squad of Elven snipers at them. The three best ways to stop a Spiked Chain trip monkey is to use monsters resistant to trip, ranged attacks, and larger creatures (a common feature in most campaigns anyway). Or, another Spiked Chain wielder, for that matter. Overall, I DO allow Spiked Chains, but if it becomes a problem, the monsters MIGHT get smarter and start to use ranged attacks.
As for a good Spiked Chain wielder, consider the Goliath. Powerful Build increases trip effectiveness and weapon sizes, and they have no racial penalty to Int, making them ideal for a Spiked Chain wielder. Try these stats out, for a Fighter 9/Barbarian 1 Goliath put into MM format for your viewing pleasure.
[sblock=Goliath Fighter/Barbarian]
Goliath, 9th-Level Fighter/1st-Level Barbarian
Medium Monstrous Humanoid
Hit Dice: 9d10+36 plus 1d12+4 (101 hp)
Initiative: +1
Speed: 30 ft. (4 squares)
Armor Class: 19 (+7 +2 breastplate, +1 amulet of natural armor +1, +1 Dex)
Base Attack/Grapple: +9/+19
Attack: +18 +2 large spiked chain (2d6+10/19-20) or +11 large masterwork composite longbow (+6 Str) (2d6+6/x3)
Full Attack: +18/+13 +2 large spiked chain (2d6+10/19-20) or +11/+6 large masterwork composite longbow (+6 Str) (2d6+6/x3)
Space/Reach: 5 ft./5 ft. (10 ft. with spiked chain)
Special Attacks: Mountain rage
Special Qualities: Acclimated, fast movement 10', mountain movement, powerful build
Saves: Fort +12, Ref +2, Will +1
Abilities: Str 22, Dex 12, Con 18, Int 14, Wis 8, Cha 8
Skills: Your choice of 60 skill points, preferably in Intimidate, physical skills, and perception skills. Keep in mind the +2 to Sense Motive.
Feats: Combat Expertise, Combat Reflexes, Exotic Weapon Proficiency (spiked chain), Improved Critical (spiked chain), Improved Disarm, Improved Trip, Power Attack, Weapon Focus (spiked chain), Weapon Specialization (spiked chain)
Environment: Mountains (any)
Organization: Solitary
Challenge Rating: 11
Treasure: Standard
Alignment: Chaotic Evil
Advancement: By character class
Level Adjustment: +1[/sblock]
This should be pretty accurate (I used equipment as detailed for NPCs in DMG), and it seems pretty potent. Feel free to swap out feats to munchkinize this guy as you see fit, and keep in mind that Mountain Rage bumps you up to Large size, giving you the coveted 20 ft. reach with the Spiked Chain. Hope you enjoy him, I would be frightened by him! Oh, his only major weaknessess are his abyssmal Will and Ref saves, do with these as you see fit. For an extra challenge, try fighting this guy on his home turf - high altitudes. His acclimation means he's alright, but the PCs are at a serious disadvantage.