The ATP PhotoFinder is quite similar, in that the actual receiver is a stand-alone device that is clipped onto the camera or bag strap, and is switched on all the time that photos are being taken. It also relies on accurate date and time setting to match recorded GPS location and time data with the EXIF data of each photo. Where it differs is the method of adding the location co-ordinates to the pictures. The PhotoFinder receiver slots into a base unit charger dock that has card slots for CompactFlash and SD/SDHC cards. Simply take the card out of your camera, pop it into the base unit, and the photos are automatically updated with the GPS data.
Because the receiver unit stores its data on a removable MMC Micro card, and is supplied with a 128MB card, it can potentially store a lot more GPS data than the Sony, although it is powered by an internal rechargeable battery which has a shorter duration than the AA cell of the Sony. The accuracy of the location finding was pretty much the same as the Sony unit. Apart from a few times when it failed to get a signal, it was generally no more than 10-15 metres from the exact location.
The Nikon GP-1 is a dedicated accessory which is designed to work only with certain Nikon digital SLRs, specifically the D90, D3, D700, D300, D2X, D2XS, D2HS and D200. It clips onto the camera's hot-shoe, and is connected via a cable to a special socket on the camera, which supplies the unit with power and also takes the GPS data directly from the unit and adds it to each photo as it is taken. This does mean that the GPS system is running off the camera's battery, so make sure it's fully charged before you set out. However it does make the GP-1 very easy to use, because you can just shoot as normal, and the location coordinates are added automatically, so there's no need to muck about with special software afterwards.
Of the five units, the GP-1 was the joint fastest, never taking more than a couple of seconds to achieve a satellite signal, and its overall accuracy was also very good, seldom more than a few metres off. It is a small and very lightweight device, so it doesn't affect the handling of the camera, and it is fairly robust and attaches securely to the camera. It's certainly the most professional of the devices I used for this trip.