I also think the batteries are worth the modest cost. In my experience, they basically work. But I have seen credible reports that they might test out at a lower capacity than claimed -- 1200 maH vs the 1500 maH nominal rating. The quality can vary from unit to unit.
One other caveat, I think, is that switching batteries with mismatched capacities might confuse the calibration of the battery meter on the phone. So the percentage reported, which is really an estimate based on empirical measures of voltage and temperature, may be a little off.
But compared to the high price of an OEM spare, the knockoffs seem like a thinkable method to mitigate battery issues.