Yes, definitely. We cruised a 3-night on the Magic last year, and have wonderful memories. We just finished a 7-night on the Fantasy, and I loved being on her for a week, but am also looking forward to a 3-night double dip CC getaway on the Fantasy in May. We have a 4-night booked on the Dream next December, as well- my son would like to try all the different ships.
In general, longer is better, but a vacation doesn't have to be long to be great. As a person with a high-stress lifestyle and tight schedule, I'd rather take more frequent, short vacations than less frequent, long vacations. That way there is always a trip on the horizon to look forward to, and a fresh memory to reflect on.