Its one of my favorite civilizations to play. Cant really give a good answer to why, but at least a few good arguments can be made. Posted some in no specific order:
1. Creative is bloody great in the higher difficulty levels. Land is power and you can block it of nicely with creative.
2. Expansionist: Pretty good too. All the cheap buildings aside (good bonus just that) I always tend to have bigger problems getting enough health than happiness in my cities (I chop a lot + HR and many more happy-giving-buildings than health-giving ones are the reason why).
3. As already mentioned the UB and UU lies on the same path and is easily enabled:
It also all nicely beelines together for the Khmer:
BE's come with Construction, which also gives Catapults and half-price Collosseums. Construction follows from Mathematics, which gives the Khmer UB, and Writing, which gives half-price Libraries. Machinery is a bit more off-target, although you also need Iron Working, which is a prerequisite for Compass, which gives you yet another half-price building.
4. UU can be great if you get attacked and have to play defensively (much more likely on higher levels, but if your really good you should not get attacked and then it kinda suck I know...).
5. The starting techs are just bloody great!! Mining everyone knows why. Hunting gives you the ability to research animal husbandry directly if no farm-based resource is near (worker first can actually be good once in a while) and you get archery quickly if needed, again great on higher difficulties.
6. Last but not least you get to play the most alien looking leader there is knowing you must be scaring the




out of all the other ai:s.
So lets all go home from work and start playing some civ4 with the khmers!
Edit: apparently some decent swearwords are censored out with




.. .nice feature, bad me.