Just got my G2x today. When I inserted the battery into the phone it showed like a 20% charge on it. So I plugged it in for a few hours before powering up (finally powered up when it looked like it was about 90% full. Then just a little while later I noticed a note had popped up saying, "battery fully charged, please unplug to conserve power". So I unplugged it and away I went.
That charge lasted me 8 hours and 5 minutes before the device shut itself off. Now mind you, being my first day with it, it barely had 5 minutes of peace before I was picking it back up again and playing around on it. So that's a hair over 8 hours of very, very heavy use.
That's DEFINATELY a lot better than what my stock Droid X battery gave me. Granted that may have been a faulty battery for all I know, but when I first got that phone I was coming off a Blackberry Curve that I had to charge maybe once every two days. So naturally when I opened the box it came in, charged the battery up to full, and had it die on me about 4 and a half hours after I unplugged it (keep in mind that was also extremely heavy use), I was anything but a satisfied customer.... I damn near packaged it back up and sent it back to Motorola.
But the phone had captured my imagination by then, so I ran out and bought the extended battery for it instead (take that Motorola! lol). Which did a MUCH better job keeping it juiced up than the stocker did. Money well spent!
I wonder if they have an extended battery ready to release for the G2x yet?