The problem I have writing on tablets is the on-screen keyboard just can't compete with a physical keyboard that offers solid tactile feedback. It is just too easy to make an error on a touch screen where I rarely would with a keyboard. For short pieces, this is not a serious issue, but it becomes an annoyance that increases exponentially for me the longer I type. There is no way I could ever type as quickly on a virtual keyboard as I can on a physical keyboard, incidentially. 

You can add a keyboard, as suggested by the previous poster. If you get a good one with physical keys (not the membrane kind), it negates much of the problem. However, at this point, if you carry the keyboard around all the time, you have essentially created a mini-laptop (or perhaps more accurately a netbook) with a small touchscreen and no mouse, which to me kind of defeats the purpose.

On a  more positive note, there are things such as the very nice but relatively small display, as well as some workable word processing software, including Documents to Go.  

So, is an iPad a good choice for doing a lot of writing? I suppose I would say it is an ok choice, but compromised.  Then again, maybe I'm just getting old and set in my ways.