It seems that the best extended battery for the Bionic is the one made by Motorola in that the software plays nicely with it. I use the Bada$$ Battery Monitor (substitute 'ss' for '$$') as FoxKat recommends and it's accurate with the Motorola extended battery.
When I had my OG Droid, I had a Seidio extended battery. No app would give a real reading, even Battery Left, which had a setting for the Seidio. In the end I used the Battery Left widget, but as it could give 3 lines of information, I put Voltage on top. I knew that if it was 4200mv it was charged and around 3500mv it was time to find a charger.