Don't expect your auto navigation system to replace a 'co pilot' by any means but it should definitely do in a pinch and be more than capable of helping you get through a few lost moments while stuck in traffic, detouring, or taking a country drive. You won't have a natural conversation with your navigational system no matter how much the television commercials attempt to convince you that this will be the case. You can however get great details about every single turn of your trip including details about landmarks you should see along the way.
You should be aware that when you miss a turn the alternate route isn't immediately forthcoming. Computers contain data and it takes a moment to go through the mountain of data sometimes in order to come up with an alternate route. Be patient when dealing with your auto navigation system and it should serve you well both in times of crisis and during your regular daily driving. I know this isn't always too terribly easy in high stress situations but when you look at it, using a navigation system is a far cry better than relying on maps that must be dug out, dusted off, and translated or an inability to read a map and ending up even more lost than you were to begin with.
Even the best auto navigation system on the market isn't a replacement for attention to details, seeing and remembering important landmarks, and paying attention to street signs and what lies ahead. The good news is that the technology for these systems is constantly evolving and improving. Data is synching much more rapidly than ever before and more and more often you are finding much more intuitive and user-friendly interfaces. The maps that are provided onscreen (in the more sophisticated versions) combined with the verbal directions and details work together in order to create a great driving experience, which would otherwise be filled with uncertainty, doubt, and no small degree of stress.
Regardless of how effective the auto navigation system you choose is, it only matters how effective you feel it is. If it gives you the directions and insights you need in order to reach your destination safely and on time, then I really can't find fault in the system and would hope that you would not either. There is no such thing as problem free in this day and age and these systems are only as good as the mapping data that they have. The really good news, however, is that data is more often accurate than inaccurate and in most cases you can easily reach your destination.
When selecting an auto navigation system you should really consider how often you travel and how often during that travel you wish you had an alternate route or at the very least an option for your directions, how often you find yourself lost and clueless and needing to ask for directions, and how often you really feel you will use the device you decide to purchase. If you don't feel you will get the most possible use of your navigation device, then consider one that can be used outside the automobile as well or at the very least one of the less expensive versions. You want to get your money's worth from whichever device you choose for your auto navigation system.