iPrinter turns your iPad into a printer. Your iPad will show up in the "add printer" dialog on your computer (as long as yoi're on the same wifi network.) Once you've configured it (by clicking a few buttons) you can print right to it to it the same way you'd print to a physical printer. You'll see your printouts on the iPad, though, and you can feel smug about wasting a little electricity instead of a lot of paper and ink.
There are lots of PDF readers (with some marvellously complicated ways of getting docs to your device.) There are plenty of apps that will let you print from your device to a physical printer, which makes sense if you have only a bitty little screen.
Unlike the iPhone and iPod Touch, though, the iPad is a joy to read from. You won't spend all your reading time searching for your files, figuring out which PDF reader you left them in, and then pinching, zooming and panning around once you do find them.
The iPad is meant to be a printer, and we're happy to make that happen.
You need a computer to print from and a wifi network to connect to.
Because we wanted it to be simple. Not just "go to the connect menu... oh, now find your IP address... OK, fire up FTP" simple, but dead simple. "Hit print" simple.
Sure, you could print all your documents to Preview, save them as PDFs, and drag them over to the iPad and then read them. That's a few extra steps, though, and if you're as lazy as I am, it might make the difference between reading the documentation, or just jumping into building your portable thermonuclear device without bothering with it.
You do have to add the iPad as a printer (just like you'd add any physical printer.) Once you do that, you can just print to it. Files will stay in the print queue if the app is not open, and print once you start up iPrinter.
If you use a VPN, in most cases you won't be able to use iPrinter while you are logged on. (For geeks: if the VPN forwards all traffic. Some don't, most do.) You won't be able to use any physical printers on your home wifi network at that time, either.
Using Bonjour for Windows may seriously exercise the fast-button-pressing skills you aquired during your FPS days. The dialog boxes come up and go away really fast. In the next version, due out in June, we will be supporting zeroconf, making installing a little easier for you PC folks (your lives are tough enough as it is.)
I always have something lying around that I need to read. I print long emails, reference docs, readme files, ebooks, anything that would normally go into the reading pile. I can also print from programs like Photoshop that I can't carry around. I can read my printouts even if I'm someplace that has no wifi.
If you've ever been at a meeting where someone says "let me print you a copy" - well, now you don't need to waste their paper or ink.
If I had a time machine too? I think sometime in the 80's, when I was supporting a mainframe and had to read a lot of SLIP dumps and traces and other great long files that ended up printed onto pounds of stripey paper with holes in the sides.