I am using this regularly to georeference lots of photos. I use a Garmin GPS and save the tracks, and then georeference from the tracks. There are lots of features of this app that I have not used, but the ones I do use just work. However, I have found, after georeferencing thousands of photos, a few gripes.
1. If you georeference from GPS tracks, you have to remember to set your camera's clock before you start taking photos. If you end up changing the clock after you start, you need to set up two documents--one for before and one for after changing the clock. (You better write down when you changed it!) Likewise, if you have 2 cameras, you need to set up separate documents for each camera unless they are exactly synchronized--which is clunky (the delta between camera and GPS time is constant for each document). A better interface would be to have the delta default to the same, but be able to change it for groups of photos. Then you could keep all the images from the same trip, but different cameras, in the same document.
If you are GPSing from Google Earth, the interface is confusing. If you click on an image that has been georeferenced, then Google Earth is activated, and your next keystroke goes to Google Earth. But it hasn't been georeferenced, then Houdahgeo keeps control. This is very strange behavior--you run through a bunch of photos trying to remember where you took them, and use the down-arrow to scroll through them. When you reach an image that has been georeferenced, suddenly the scrolling stops, because Google Earth is now getting the keystrokes.
This is a file-based program (which is probably right), so it saves a reference to photos and various data for each photo (like Lat/Lon) in the file. You then write out the data when you are done (so that, e.g., the Lat/Lon will be included when the photo is uploaded to Flickr). However, there is no indication within the program whether the Lat/Lon in the image is the same as in the Houdahgeo file. So it is very easy to update the Lat/Lon in Houdahgeo and forget to export the change, so your images have the wrong data.
Overall, a stable and very useful program. The developers have done lots of work to make sure it interfaces as well as possible with various photo programs like Lightroom and iPhoto (which I don't use). But it could use a complete review of its user interface to make easier to understand.
Also, I recommend that you become familiar with exiftool, as it is the easiest way to change the date/time in a photo if you set your camera's clock incorrectly, which you should do before you run Houdahgeo.