I've heard people talking about Anker on big forums about Anker battery, basically cell phone batteries and external batteries, and it seems people speak highly about it. So have you have any idea about how it works?
I'll assume you actually want to know what ways are there to increase battery capacity of your Android device. There're three approaches to this problem.
Approach one is an internal battery with higher capacity. I don't have any details on whether Anker is worthwhile, but it's capacity for say HTC Desire S is 1600 mAh and standard battery capacity for the same HTC Desire S is 1450 mAh, so that doesn't look like a huge improvement to me - it will be drained in one day of usual use anyway. What is really important is battery reliability - you want a reliable battery with long lifetime (number of charge-discharge cycles and capacity retainment) and good safety so that it doesn't explode in your phone (that actually happens once in a while with cheap batteries) and you have to look for such information elsewhere, not on this site.
Approach two is a spare battery. It's not very convenient - phones are not designed to allow for fast battery swap and phones lose power and their state when a battery is swapped and also charging the spare is a problem, because you either need a charger craddle for that or you use your phone for that and the latter means more battery swapping.
Approach three is an external battery - there's a lot of such stuff, Anker sells that too, they come in a wide range of capacity, some can even be used with netbooks. Basically it's a box with a battery inside and a USB cable. You charge the box either of USB (your computer, laptop, USB hub, phone charger, whatever) or of mains - that depends on the model. Then you can use the box to power any USB device, phone included - the phone will think it is connected to USB and charge through it. This is a rather convenient option - you can temporarily connect your device to the box and charge it fully or partially, you can carry it conveniently in a bag or a pocket and have the device connected to it and use the device at the same time. One big advantage of this approach is that the box can be used with various devices, not a specific model only. One big disadvantage is the thing is relatively expensive. Again you need good reliability, lifetime and safety, you should search for that information elsewhere. Another disadvantage is that the box itself can be slow to charge - its battery capacity is usually rather high and USB doesn't provide lot of power, so charging current will be rather low, this might be important for your convenience.