Is circuit training most beneficial if you do it everyday? Should you take days off or alternate with something else to get better results?
It depends on the circuit you do. You could alternate a circuit such that you don't do the same muscle group on consecutive days if you like working out every day. If you are working your muscles hard, you definitely want a 48-hour recovery period for each muscle group.
I did a pretty intense circuit program 3 days a week (M-W-F) so I could have weekends free. And if you do it right, you sure appreciate those days off. But sometimes due to scheduling, I've done circuits consecutively. So long as you know your limits and keep good form, you can avoid injury.
You can always do another activity for days in between. I would suggest if you worked legs the day prior to not run a really long distance the next day. A lower level of intensity like a leisurely walk, swim, or bike ride is nice but it's all relative to your activity level. Listen to your body.
Since circuit training is a form of strength training (strength endurance), you should only do it 2-3 times per week with a day or two in between. Failing to take time off will cause you to reach a plateau due to the fact that your skeletal muscles will develop faster than your internal organs can keep up.