I got a 3GS as my first iPhone and I've been very happy with it overall.
One thing that caught me off guard was how complicated it is to save a file "somewhere" on the phone. On my WM phone, I could right click an email attachment and save it on the phone. It had an Explorer roughly equivalent to the OS X Finder and you could copy, move, cut and paste files at will. There is no Finder on the iPhone. And the iPhone does not show up as a drive in the OS X Finder. You can't transfer a file to it via USB. Storing a document such as a pdf on it requires an awkward 3rd party app and a WiFi connection. It's not a big enough problem to be a deal breaker, but it's something that caught me off guard.
And once you do get the document saved via WiFi, don't expect to be able to select it as an attachment and email it to someone else...
Another obstacle I didn't anticipate was not being able to edit Word and Excel documents without a third party app. You can read them but can't edit them. But since it's such a hassle to save a document on the phone to begin with, you probably won't be editing too many Office docs anyway. And if you do edit them, don't assume you'll be able to email the revised doc to anyone.
Other than those minor issues, I think it's the coolest phone ever! Overall, I'd say go for it.