I ama long time Pocket Informant user. I've used it on Windows Mobile and one of the reasons I bought the iPhone 3G 2 years ago was because Pocket Informant was coming. It did take a littlelonger than they planned to get it on the iPhone, but once it did I have been using it since. Therefore as soon as I could get it for the iPad I had to have it. I was already using the iPhone version on the iPad anyway.
I never used Google Calendars until I got Pocket Informant for the iPhone, and quite frankly it's really pretty good. Combined with Toodledo, and the fact that Pocket Informant allows you to use the Franklin Covey method of tax prioritization over the over rated GTD method make PI a winner.
MobileMe calendars only allow me to have 6 calendars. Google is more or less unlimited. I use the calendars like categories there is one for work, one for personal, one for special events, one for family and friends, one for television shows I want to see, one for workouts etc. I could easily pair them down to 6 for use in MobileMe and do for syncing the built in calendars of the iPad and iPhone, but with Google/Pocket Informant I can get a little more specific. On top of that, I am not limited to the 6 colors that Mobile Me and the iOS calendars present. Pocket Informant allows me to customize my calendar color to an color I want, and Google offers over 20 colors as well, which is more than I need since I really only have about 12 calendar (categories)
Right now I am happy using both ways. One in a sense could back up the other.