I haven't used straights in a long time now. I use circs for all my knitting except sock knitting, for which I use DPN's. I find that you can knit faster and easier with circs, they don't take up as much room, when knitting or when storing your knitting. It is more comfortable to knit in the car with circs. It is easier on your wrists to use circs because the weight of the knitting is in your lap not being held up by your wrists. I think you'll like them. You use them just as if they were straights; knit, turn, purl, etc.
If you decide you like circs, you may even want to look into getting a set of interchangables.
Good luck with your afghan!