I've been using the 1500mAh battery for a week now, and have noted a real improvement over the stock offering. To echo what's been posted...3-4 full charges are recommended before you see the potential realized. A very real 20% gain in battery life is possible, depending on your level of usage.
I will have a full review of this, as well as a couple of other 83xx Curve-specific products posted in the next day or so.