[font color=\'#000000\']I, too, use after market batteries - the 511's - for my D30. They work well.
The thing to look for is milliamps (mAh or something). The more the better. Personnally, I love having 1500mAh.
I've had my Canon battery for 3 years. I then got two aftermarket batteries for less than the price of 1 Canon, and they last longer due to the amperage. After not using my Canon for a month, it's now worthless - doesn't keep a charge. Can shoot 15 minutes before the battery warnings go off. But, it did last 3 years! And I haven't had my aftermarket ones long enough to compare lifespan.[/font]