Certainly, the easiest way to place a KML file in a Google Map is to use GGeoXml, as described in the API documentation. GGeoXml reads style inforation from the KML file to automatically color lines and polygons as indicated in the file. Furthermore, GGeoXml encodes polylines and polygons for efficient rendering, as well, and it's written by Google programmers so it must be good!
There are two main issues with GGeoXml, however. First, there is no way to easily change the styles defined in the file. Second, and more to the point here, there is overhead in the process that can be avoided by encoding the data first.
The map below shows driving directions map again, as generated by GGeoXml. Note that it is a bit more sluggish to load, as compared to our properly encoded example. Of course, this is because the entire (large and unencoded) KML file must be sent to Google's server and then encoded before the encoded path can be sent to the browser. These first two steps are already done by pre-encoding the path.