As I suspected, when used away from the interference of city centres, it locks on to the satellites much more quickly. Not only that, but in open areas like sea fronts, zoos and the like, the accuracy of the device (based on plancements on Google Maps) is pretty damn excellent. Not just “close enough to remind you where the picture was taken” but actually close enough to be very informative.
While we were at the zoo, the battery light started to blink – this was after a lot of use, so I wasn’t all that surprised. I couldn’t tell you how many hours the device had been turned on, but it was quite a lot, and much more than I’d normally have it on. I’m pretty sure this was just a warning rather than an OMG! Change now!!! but I changed the battery to avoid it dying when I wasn’t watching the lights.
So, yes. It works well, does what it says on the tin, and is much more convenient than using a separate GPS device and having to fiddle with software. If you have a 5D Mark III or a 7D, this is well worth considering, unless geotagging your pictures is of no interest to you. Personally, I’d hate not to have my images tagged – it’s so much easier when you come to look at older images and you can’t quite remember which bit of coastline that is. A few years back, I had to resort to examining the coast in Google Maps and matching what I could see there to my images, which took a while.