I began my Masters immediately after completing undergrad.
I'm glad I started right away because I believe that, had I waited, I would have slacked and not wanted to go back.
Don't get me wrong, it is difficult. I work full-time, so I can only take 2 classes, both in the evenings after work. Most days I'm exhausted. The course-work is less than appealing and the cost is painful... lol... but as many of my friends and Professors wisely advised... Do it now while you're in "school-mode." You know you're going to go back... might as well do it now. It will be harder later. Once you get a taste of the freedom of not having to do papers every week, it's hard to go back.
Of course, everyone is different. But I believe in continuing right after undergrad.
