I only have second hand experience of iphones, my friend has one, but I would totally agree with the warnings about the compass. On a recent trip to Edinburgh his iphone led us on some merry trips to undesired, and undesirable, locations thanks to the dodgy compass. We eventually agreed that it was trying to get itself stolen
As for useful apps, I recently heard from someone about a project to make a website where people can log locations of plant and tree species on an online map as a kind of wiki. Eventually it could build up into a really useful resource. I haven't actually looked into it yet but that would be a extremely useful tool to have when out and about.
Edit:
I've just had a quick look on the web and found this
http://locations.sprouting.org/
I'm not sure if it's the one I was tolled about. I got the impression that that one was still in the planning stage, a university project i think. But this looks like the same idea if it isn't the same one. Could be one to watch.