Initially these seem to be fantastic, the ability to play and charge meaning minimum interruption for avid online gamers. Sadly the performance soon drops off. I own two official battery packs and one third party one, I also own both an official and third party USB charging cable.
The first third party combination gave me well over a year of daily usage but naturally the battery life shortened per charge, this is to be expected. I bought an official battery pack and within a month of usage it was giving erratic readings and not always charging when the USB cable was connected. I then bought an official cable only to find it made no odds to the erratic behaviour, fortunately I'd got a new battery with the cable so started using this instead. Again only a few months later the battery warning would light up every other day or so. The battery doesn't always start charging when the cable is connected, occasionally needing disconnection and reconnection. When the battery is charging it rarely detects the battery is charged and often the battery becomes excessively hot. Sometimes powering off the console still leaves the battery charging for a short while before registering the battery as charged. On occasion it's not registered and continued to charge for some 5 or so hours after powering down. Upon powering up the console and repeatedly unplugging and reconnecting the charger it will miraculously register as charged. I have three controllers and get the same behaviour regardless of which controller I use. Half of my time playing I have the charger connected so I may just as well be using a wired controller.
Incidentally, in the first couple of years I was using my 360 above 7 hours a day. These days it's more like 3 hours per day so the need to charge the battery pack every couple of days seems ridiculous, especially as often my gaming session is shorter than the indicated time needed to charge the battery pack.
Reading gaming related forums it seems that others have similar experiences. The general consensus being that it's better to use traditional AA rechargeable batteries on rotation or just use a wired controller.