I'm not a medical expert, so take my comments with a grain of salt (then again, most sites and individuals advocating jelqing aren't medical experts themselves, so there's that).

As of today, there have been no scientific studies showing any relationship between jelqing and an increase in penis size (mostly because few studies on the subject have been done at all). Furthermore, I've read that the "logic" behind jelqing--something like "stimulating the penis with certain pulling/milking motions stimulates the blood vessels and encourages growth"--is based on absolutely nothing; blood vessels just don't work or "grow" that way. And finally, yes, I've heard that jelqing can lead to ED; however, I'm somewhat skeptical about this claim as well considering the overall lack of studies on jelqing. I suppose the risks involve depend a lot on how rigorously you're jelqing. If we're talking basically something like a massage for your penis, then that's probably not going to be particularly harmful. But if you're grabbing your junk in a death grip and mashing/pulling it hard, yeah, that will definitely cause some problems.