I don't have an iPhone but an iPod instead with no inbuilt GPS engine. The Garmin Glo GPS Bluetooth reciever works very nicely with the iPhone mapping system apps on the iPod.

What is important to improve GPS accuracy is HDOP and VDOP which is related to the Sky View and number of Satellites. You could for example put the Garmin Glo under your hat or in a top pouch in your Rucksack. This will improve the number of Satellites your GPS antenna/engine will see.

The Garmin Glo GPS is specified to pick up GPS and GLONASS signals from both constellations and will typically see around 12-18 satellites rather than just the 6-9 GPS NAVSTAR satellites. This could be useful if the NAVSTAR constellation system goes down.

The update rate is also 10 times that of conventional GPS engines.

Maximum Accuracy would typically be;

GPS - 8-10 metres with Selective Availability turned off

GPS + (WAAS or EGNOS) - 5-8 metres

GPS + (WAAS or EGNOS) + GLONASS i.e. the Garmin Glo - 2-3 metres

DGPS - 1-2 metres depending on baseline distance

RTK GPS (phase measurement with small baseline distances) - 0.01-0.02 metres

In the UK the mapping accuracy for OSGB36 grid mapping is typically 2-3 metres for a OSGB map triangulation point so DPGS and RTK GPS would be rather pointless for general navigation.