Extended Battery case, is it really worth it????

[dankomba]

I'm considering to buy Ultra-thin external battery case for my Samsung Galaxy S2 i9100 on ebay below.

Aside from benefits of having extra usage of the phone, what are the downside?

Did anyone bought these similar products? How much extra usage time did you get and how heavier and longer is your phone? Is it really worth it?

One is 2800mAh battery and 2200mAh battery, what other benefits do you get other than longer usage? How heavier is the phone?

2800mAh battery:
2800MAH-ULTRA THIN External Backup Battery Case for Samsung Galaxy S2 II i9100 | eBay

2200mAh battery:
Ultra-Thin Extended Battery Power Pack Case Samsung Galaxy SII S2 i9100, 2200mAh | eBay