I type about 90wpm on a keyboard and I can get a pretty consistent 55wpm - 60wpm using an ipad iwth the touch screen keyboard.
I've used it as a big part of a content creation when creating with training materials and writing sop's, etc.
It's obviously slower than my normal typing speed, but I've actually been surprised at how content I've been using it for that sort of thing. If I'm authoring anything of any substance, the bottleneck is usually my brain and not my typing speed, as far as phrasing and structure goes. I had originally thought about getting an external keyboard/keyboard case for it, but once I realized I could bang out text pretty quickly, I said "screw it" and just got a smart cover. I also hated the idea of bulking the thing up.
I've also done a lot of document notation and review with PDFExpert adn that experience was really handy. Obviously you wouldn't make the final edits there, but given the right software, I don't think it would be that crazy.
Obviously hugely subjective and ymmv
I've also wondered how much could be done with a touch screen-only interface for code editors, using structure detection and the sort to input elements.
After using Lisping for a bit on the iPad, I think you could really do some interesting things to make a really productive editor on an ipad. I really like that project because it seems to be the only editor I've found that really tries to embrace the tablet experience from the ground up. It's not perfect, but it's definitely a step. Not sure there's a market there, but it's interesting to me.