I tend to knit everything on circular needles (they're so much smaller and more comfortable in my hands). Plus, like other posters have said, it can be awfully hard to get all that knitting to stay on one straight needle at a time.
I was given the delightful AddiClick system for Christmas and can't tell you how delicious they are! They never come unscrewed or wiggle, they're perfectly seamless and I think the nickel-plating might just make me go faster. I am in love with them, but since I do knit mostly vintage patterns I have had to purchase a couple pairs of itty bitty needles.