I use 40'' but they really don't have to be that long. I just buy the big ones, so if I need longer ones later, I have them.
I used to use DPNs. They are all very lonely now. I was going on my honeymoon and wanted to bring my knitting on the plane. I was afraid of bringing 5 very pointy metal objects on the plane and decided to learn magic loop. I just looked up how to do it on the Internet (this site included); it's just so much faster, and I no longer have ladders where the joins are. I've found that my stiches are much more even now. I also found this to be true when I was working on a pair of socks that used lace. I find that it is really much easier, and I don't "accidently" pull out the wrong needle while knitting.