This version is largely dedicated to supporting outdoor athletes and their GPS devices (hikers, bikers, runners, etc), but comes with a few other Goodies as well.
You've long been able to import GPS data to Google Earth, but 5.2 improves the way that it is handled and displayed. There are two main features that have been added to this version:
Up to this point, GPS data had to be entered as either a series of line strings or hundreds of points. This new KML extension is a much smoother alternative, and is perfect for importing a GPS track. It contains all of the GPS data from your device; time stamps, lat/lon, heartrate, etc. The playback is similar to the "hundreds of points" display from before, but with a much smoother playback and a less cluttered KML file (both in the code and in the "my places" display).
Elevation Profiles: As part of this, you can now pull up an "elevation profile" of your track. This allows you to see the elevation gains/losses throughout the duration of your GPS track. If you want, you can zoom into a particular point in your track and just view that data (when climbing a large mountain, for example). Even better, the elevation profile can be generated from any linestring so you can use this feature on your existing KML files if you'd like.