I bought two of those batteries+charger off of Ebay for my Gplayer, and I am duly impressed at the life they have for only $2.50 apiece. I have been using them for about a month now, power cycled them several times initially to break them in. I get what seems to be 95-115% charge of the stock battery, after a week or so. I haven't done any scientific comparisons, but I dragged my stock battery out yesterday, and it seemed to drain much faster than these replacements. The build quality is horrible, they fit loosely in the tray (but don't pop up/out when the case is on), and I wouldn't expect them to last more than a year at most, but for a spare to carry around on a long day, or as an emergency backup, these are an amazing deal for 10x less than equivalent replacements.
Side note: I do not believe these batteries output as much current as the stock battery does. I am heavily oc'ed/ov'ed (1.5GHZ 1.4v Core/1.19v Internal), and that seems to be that max I can push it, because if I up the voltage any more, the Gplayer reboots. This doesn't happen on the stock battery, so people who aren't overvolting/overclocking to insane numbers shouldn't have to worry about it.