Hello Jim,
I'm a fairly new/inexperienced pd user, and definitely not a programmer, but I've been doing great with pd. Here are my answers to your questions:
1)I've used it to study some basic aspects of sound synthesis, to make some interesting sounds I might use in future pieces, and recently in a piece for snare drum and live electronics. Regarding the recent usability thread, I must say I adapted very quickly to the pd environment. I don't think anyone wanting to use it for what it does, not only as a collection of someone else's patches, will be scared of its interface (which isn't to say it can't improve).
2)As far as I know, pd is not the best tool for soundfile editing... but that wasn't what I was looking for at all.
3)My favorite resources are this list and pd.org. I find all the links I want from these two starting points.
There are two things everybody here knows but that I always stress when talking about pd to people who don't know it yet, and they are among the strongest points I see in this software: it's free, hence constantly getting better by everyone's contributions, and it will run on virtually any machine. I think these are the reasons I won't stop using pd even if I get a Titanium 1GHz with Max on it.
Cheers, Guilherme