Just got this bad boy delivered today and so far, it works great and it has slot to charge a second battery but unfortunately not an extended battery. It does have a insert in the main cradle however that can be removed so you can charge the phone with certain cases and possibly an extended battery (Have not tested yet).
Can charge the phone and another Battery at the same time.
Has a good weight and is very stable.
The USB wall charger it comes with does not give you a full AC charge but a USB charge. I changed out the wall charger and I was back to AC.
Can't charge the extended battery on it's own. (you might be able to do some cutting in order to make it fit however.)
EDIT**** I know I said that this will charge an extended battery but I'm not sure that's fact yet. I believe it will only fit certain cases and not sure if there is enough room for the extended battery. If after testing it does fit, I'll make the corrections here. Sorry if I got any false hopes up.