I bought my first DSLR (Canon 20D) back in 2005. Genuine Canon batteries at the time were going for about $100 (give or take), but Sterlingtek were selling them for only $10 each
After reading lots of good reviews, I took the plunge and tried them out. They even had more capacity (both in mAh rating and in actual shots taken) than the genuine Canon ones!
When I upgraded to the 5DmkII, I just stuck with the genuine batteries as they're microchipped and the premium was less crazy.
I think the main concern with ultra cheap Li-ion batteries is that if the internal circuitry is dodgy, they have a habit of exploding or catching fire...