For travel writers/photographers, GPS is indispensable. No matter how good your memory, after tens of thousands of photos shot all over the world you cannot possibly recall where all of them were taken. Add-on devices, like the Canon GP-E1 for the D1X, 5D3 and 7D are expensive, bulky, add complexity, and serve a single purpose. Internal GPS, like in the 6D and Powershot S100, S110 have two downsides: they take too long to acquire satellites when you're frequently turning the camera on and off to conserve juice, and they consume more of your battery's power than the same camera body would without it. Far afield, when battery power is not easily replaced, this is a real concern.
I prefer to use a real GPS while shooting, like a Garmin CSx60, since it provides so much additional utility for navigating, setting waypoints, routing, tracking, etc, and is a far more sensitive and reliable receiver. I then use Early Innovations Photolinker software to match my daily GPS track to the day's photos. This tags each image with the appropriate coordinates in the EXIF data, shows you each image on a map, and allows for manual override when necessary (like when you're in a cave or slot canyon and loose the satellite signal). Lightroom 4 then displays each image's location on a world map once the image comes off of your memory card and into your computer.
I don't work for, am not sponsored by, or otherwise shill for any of these companies. This is just the best way that I've found to get my work done. There are a number of alternative geotagging software applications out there, but I haven't found any of them to come close to Jeff Early's Photolinker. It's fast, intuitive, comprehensive, and he regularly updates it. I hope this information proves useful.
The new GP-E2 replaces the need for a standard GPS unit for the 5DM3 unless you need it to navigate. It acquires signal lock in less than 30 seconds most of the time. I do a travel photography trip (usually to a country I'm not that familiar with) once or twice a year. I was in Scotland last June for 2 weeks. My memory is pretty good to but, 14 consecutive days of travel hitting multiple locations each day, the geo-tagging was a big help when I got home. Plus if I ever want to return to the exact location I'll have the GPS coordinates to do it...
My only complaint about the GP-E2 is that it isn't powered by the camera and needs AA batteries. I took lithium batteries with me and didn't need more than a 4-pack for the entire trip so it's not that big of a deal but it could have been made smaller if it didn't need the internal battery.