Please note that the only way to make batteries that cheap is to use the cheapest possible materials and skimp on the QC. Oh and for most of the cheapo batteries, the mAH figures are basically made up. You don't have to buy original Samsung batteries, but at least get an Anker battery or other recognized brand. Yes, they're more expensive but they probably won't blow up in your DX90, turning your DAP into a $419 paperweight. Also, I'm fairly certain that a 2100mah Anker battery will last longer than a $3 "2900 mah" battery.
You are definitely right. My HTC Vivid has one of these type of cheapo batteries and it use to last for up to 30 hours. Then less than 4 months later, the battery went down to 10 hours, maybe up to 12 hours. Go for a recognized brand and you won't go wrong.