I've been using Printopia for a few months now. I have an old Brother laser printer, a Canon Inkjet, and a new Canon MF4350D. I tried both of the other apps but found neither allowed me to use the duplex feature on either printer. With the MF4350, the other two apps only printed the first page and then stopped working! At work, some of our printers have passwords, and again only Printopia allowed me to print to those. It seems Printopia is a complete implementation of the AirPrint protocol, as all of the features work properly, while the other apps just seem to trick the iPhone into thinking it's talking to an AirPrint device when it's not- which explains the problems and the prominent disclaimers on the other apps websites. Printopia is well done, you can tell they've put a lot of work into this.