The batteries normally have a USB and provide 5v out which is enough for powering most Arduino based projects. They also contain a LiPo battery, LiPo protection/charging circuit, Solar cell and a 5v step up regulator. All of which is worth more then the $5-$10 you pay on ebay.

What you can also do with this is because it has 5volt regulated output and only a single 3.7v LiPo cell you not only can Power your next Arduino project but you can use a analog input to monitor the battery level.

The analog pin will return a low number for a low voltage so 0=0v and 1023=5v(Vcc).

If Vcc is exactly 5v (not likely) you use a trick like this to return a rough voltage

    sensorValue = analogRead(analogInPin);

    BatterytVoltage = map(sensorValue, 0, 1023, 0, 500); 

so if the battery is at 4volts, BatterytVoltage = 400

A LiPo battery fully charged will be just over 4.2v and below 3.5v when discharged. Over charging and discharging can damage the battery which is why the charging/protection circuit is required. This will let you keep and eye on the battery level and know a rough battery level. 

It is also useful to attach a resistor to the solar panel and monitor the charge rate too if you have a spare analog pin.  

This is what I am doing for my http://www.gr0b.com/0_o/updates/pyroelectricinfraredsensorspirthermograph project, I am also using the Arduino for power management to all connected devices (ie: motors, camera, lights and things to come)  

Next step is to add the wheels and send it on its way, unlike a robot cleaner that has to return to its charging dock it will just seek out a sunny spot on the floor.