I just recently sold my 1D4 and bought a 1Dx to compliment my 1Ds3. I've been thinking of buying a Canon GPS unit since I've had poor luck with several "independent" GPS units for geotagging. (As a note, I have a great geotagging app for my iPhone which I often use while scouting, but it really hammers the battery life, even though i use a Mophie Juice Pack.)
I was curious if anyone had experience with either of these Canon GPS units? The advantage of the GP-E1 is that it mounts directly into my 1Dx. But, most of my scenic work I do with my 1Ds3. (The slight edge in res is still noticeable in some shots -- but, maybe I'm just myopic from too much pixel peeping.
) Literature doesn't directly state that the hotshow unit works with the 1Ds3, but since it can operate in "logging" mode, I would think it would still work.