I have received a month ago the Canon EOS GP-E2 for my 5D Mark III and could therefore already extensively tested him. He finds himself in buildings quickly the satellite and can be configured properly on the 5D Mark III. The battery lasts long enough for even longer Photo Tours. The tracking I've also tried, what works well. In the later view can be seen, however, that some pictures have received inaccurate GPS coordinates with a few 100m deviation.
The only thing I would have liked, a smaller module would (similar to the GP-E1 from Canon), because the GP-E2 is a bit too large to put on him regularly. Therefore, a star deduction. But otherwise a perfect complement to the 5D Mark III.
