I have the Zinio app, but haven't downloaded anything besides the 3 free sample magazines they give you. From the sample National Geographic, it doesn't look like they've thought too hard about magazine layout for an iPad, because you have page cuts across a single paragraph: the paragraph is divided vertically down the middle, so you have to flip back and forth to read it. They do have a pretty wide-ranging selection. People on SF might like GQ, Esquire, Stereophile, the 3 big American car magazines, as well as a bunch of foreign magazines that may not be easy to get.
The iBook app is surprisingly good. I wasn't expecting it to be, but it's pretty addictive, especially if you're on long plane flights. I've bought and read a few books already, and have to control myself unless my iTunes bill goes through the roof. The selection's not great yet, but it's not bad, and I can only imagine it getting better.
The ABC and Netflix apps are also great. The ABC app looks like Hulu rewrapped with ABC branding, and you can watch streaming movies from Netflix. We don't think much of it these days, but it struck me the other day that I was watching relatively high-res video wirelessly on a 1.6-lbs, thin pad that's touch-controlled. It was like the future arriving: the only thing missing was a flying car in my garage.