yes, I only use circular needles.
you can make anything with a long circular needle
I don't like single pointed needles because the stitches slip off, and you have to scrunch up the yarn and it's so easy to lose a needle (I am not the most organized person in the world and if I had to find the other needle every time I wanted to start a project, I would just stop knitting right now)
also, with the circular needles, once you have a lot of work done (example: you've got a large sweater worked up to the armhole) the fabric just goes right in your lap and you don't have so much unwanted weight pulling on your needles-which can affect your gauge (and you don't want that)
and one more thing, the smaller circular needle tips (the actual knitting needle on the circular needle) is about 4.5-5 inches, which fits in your hand MUCH better than an awkward 14" metal stick.
I would suggest (if you are still new to knitting and don't have a lot of needles) just buy circulars from now on... ooh and they are much easier to store- another plus.
but one thing, if you get a circular needle with a stiff cable wand very little flexibility (some companies have very stiff cables) make sure you dip the cable on the circular needle into some hot or boiling water (don't worry, it won't melt) and the cable will straighten out and become more flexible.