My husband and I travel daily for his job. A GPS is very helpful in cities. However I will say that he will look up directions at google or yahoo maps and prints them, plus we run with two GPS's (different brands). Some days, they both will have the same directions, other days one will send you the longest way possible.
I have a gps on my phone. It is the most reliable because it is always current on maps, plus it will give traffic alerts.
I like using one when I travel, even if I know where I am going, because it will give me an idea of the arrival time.
ETA: What is exciting is when it loses the satellite signal when you don't have a clue how to get where you are going.