I have been using skyscape applications on my Moto Q with MS mobile 6.0 since I switched over from my palm treo two years ago.
They have a nice software setup where you can download different content and it cross-references between programs (If you read about a drug indicated for a disease in a referrence text, you can click on the drug name and it will take you the drug database you have purchased).
I didnt switch over to the initial iphone release because they didnt allow for 3rd party apps. On the treo, I had harrisons, pkt med, wash u, archmides, etc etc. It was difficult to find 'cheap' software for the moto, so I went with skyscape and had to pick the ones i needed the most. With the iphone, most of the residents working with me could only access online programs, usually just epocrates, but when they didnt have internet connection they were SOL.
Now it seems Apple is allowing more developers the code to write programs for the iphone and skyscape is trumpeting themselves as being one of the first to do so. Most of their programs are available already for iphone as online access, with free trials to see if it works on your phone. They are working on downloadable programs but dont have em yet. The only bad thing is the price, which seems to be the same cost as the reference in paper format. You can always charge it to your bookfund if you are lucky enough to have one.
I was all set to buy the iphone until I heard that RIM is releasing the Blackberry thunder aka 9500 later this year which might be a iphone beater and available on my current carrier Verizon. So....I think I will wait and see how people fare with the iphone and new programs...
Hope that helps...