This is an age-old discussion... Google Maps offers driving directions,
but it is NOT a navigator... that is, it does not offer turn-by-turn updates, spoken or not.
The reason is rumored to mostly be licensing... Google paid for the right to use map data supplied by third parties, but part of the licensing deal was that they are not allowed to offer a navigation service with it.
The result is that they will show you were to go, and show you a blinking dot that happens to tell you were you are, but the directions do no update as you drive. You have to figure out how far you are from the next turn, etc. on your own. Its basically tantamount to printing the directions out from your PC beforehand and following it while driving. Not the same as having an in-dash GPS as any GPS user will tell you.
In fact, the only advantage to having a GPS function in Google Maps is that you don't have to tell it your starting location.
If you want a
Navigator, then yes, you will need one of the apps wraith79 mentioned, that usually includes spoken directions and works exactly like a standalone GPS navigator.
Bing, on the other hand, is a perfectly acceptable alternative that uses your data connection (just like Google Maps). They have licensed maps that allow them to use it for navigation, so navigate it does! I actually find it more accurate and up to date than iGo and some of the other paid apps, since the data is LIVE in the cloud. Microsoft kept the UI pretty simple, but if you pay attention to it, it updates in real time and tells you how much farther until the next turn and when to turn (no voices at all, just beeps and tones).
Now, the real clincher is going to be Google's Navigator... Google's street view project, etc, has enabled them to no longer need to license maps from third parties... they now have enough mapping data of their own to do whatever they want with it. If you've seen the Navigator on the Droid, you know what I'm talking about- spoken TTS as well as speech recognition using the Goog-411 routines (Bing actually has a similar feature to recognize spoken destinations, but still no TTS).
Right now, its only in Android 2.0, but there are rumors it will roll out to other smartphones capable of Gmaps with a GPS as well.
http://www.mopocket.com/2009/10/google-navigation.php