I'm happy to report that, after an update, the voice sounds much better and is now useful. Garmin has also increased the size of the map storage buffer, so you can see more of your previously downloaded maps without having to download them again. Voice instructions can now be toggled on or off, and there's a volume control on the main map menu for easy access. There are also some unspecified bug fixes.
Garmin is to be congratulated for these quick fixes, and I have confirmed that the fixes do work and greatly improve the app. One thing that is not fixed is the requirement to download maps. While Navigon, TomTom and Magellan all have maps on board, Garmin has chosen to download maps as needed over the 3G network. This makes the app footprint small, but if you often drive in areas of weak reception, this is not the app for you.