I have an iPhone and I get by without it. Like others, it'd be nice to copy a number or address from an email and doing something with it, like pasting it into another email or addressbook (yeah, DDs would solve that last one).
What I'd actually want to see before c/p is the ability to append numbers before dialing them. If there's a phone number on a webpage or in an email, the iPhone handily makes them clickable. Of course if the number doesn't have the area code, the call doesn't go through. Without the ability to either c/p or modify numbers prior dialing, it's useless. (I've griped about this before and, no, I haven't sent in feedback.)
Twitterific does a nifty thing with its bookmarklet. It's just a javascript bookmark that passes the URL to twitterific.
Code:
javascript:window.location='twitterrific:///post?message='+escape(window.location)
I wonder if there's a way to make this type of thing work in other instances.