A Bluetooth GPS receiver is only supposed to do two things: triangulate the signals from as many geostationary global positioning satellites as it can see to work out your longitude and latitude, then transmit that grid reference as quickly and reliably as possible, via wireless Bluetooth connection to whatever device you choose.
For cycling purposes, it needs to do this quickly, it needs to be rugged, and it needs to do it all day long. The Purity performs well on all counts: it uses the latest third-generation satellite spotting chipset from Sirf (Sirf and Nemerix are the two biggest rivals in this field); it didn't lose my position, even in a steeply sided wooded Cotswold valley; and it talked continuously via a wireless Bluetooth connection from the top of my rucksack to the Viewranger digital mapping software on my handlebar-mounted Nokia 'smartphone', on and off road.
The Battery life is a claimed 15 hours and certainly lasted all day on one charge from its replaceable phone battery. For multi-day rides just buy and charge up several - supermarkets sell them now for just a few pounds. If you turn your phone off or wander out of Bluetooth range then it will power down automatically until it 'sees' the signal again. The Purity also comes with a great range of accessories, including a decent protective sleeve, a car charger and mains charger - both of which connect with the same USB cable that also enables charging from a computer. Amaryllo also produce the Amplitude, which at roughly £17 less uses the rival Nemerix chipset.