It seems like your work goes much faster when you are using circular needles. If you aren't changing colors of yarn, you can just keep going round and round until your project is completed.
When you are using regular needles, you have to stop at the end of each row and turn your work around. This is something you really don't even think about when you are knitting - until you use circular needles for the first time. If you are like me, you won't want to go back!