The $6.99US app requires that you run a print server on your Mac or PC. The server software is a free download from the developer. System requirements are Tiger or Leopard on the Mac side; a Windows PC requires XP/Vista and may require some Windows configuration.
On the iPhone side, you need to give the app your basic email information (user names, password, incoming server info) and you're pretty much ready to go.
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In addition to the features mentioned, you can access files from your desktop or laptop and print them. You tell the print server software what directory you want to access, and anything in that directory is then available. With the nearly automatic setup, the print server creates a 'We Print' folder on your desktop and anything dragged in can be printed from your iPhone back to your local or networked printer. (This is all more difficult to describe than to actually accomplish.)
Now, on to the limitations. First, if you want to print a web page, you have to type in the URL yourself. Because of restrictions on the way the iPhone OS works, Print & Share has no access to your regular Safari bookmarks. The program allows you to add several you want to use, and save them.
The iPhone's restrictions mean it's the same for Mail. You can't print from it directly, but you can set up a sort of proxy in the Print & Share application and print from there. While you can print contacts easily, you can't print your Calendar. Apple could allow this, but the iPhone is not as open as most of us would like.
The print server that runs on your PC can support multiple printers, and you can password protect everything if you like. There is quite a bit of functionality built into this app, and it's mainly held back by limitations of the iPhone and not the developers.
I tried Print & Share, and it had no problem printing contacts, emails, and images from my photo albums on my iPhone, or even documents and images sitting on my desktop computer.
I did not try remote Internet printing, but the instructions seem straightforward enough.
Frankly, this is the kind of functionality that should be built into the iPhone, not bolted on by third parties. Apple may add these features in future updates, but for now, 3rd parties are rushing in where Apple has not yet gone.