I recently had the opportunity to evaluate some students with learning disabilities in the areas of reading, writing and spelling. They were both able to use word processing programs, but spelling limited their written expression. Spell check was not able to correct many of their errors. Additionally, due to their reading difficulties, the two boys were not able to read the choices provide by spell check. I did a trial with word prediction software and text-to-speech capability and was amazed to see the improvement in their writing. The gaol was to promote independence for these learners. This article http://www.ldonline.org/article/6397 from ldonline summerizes the power of this tool. It provides researched based information for how word prediction can help learners become more indendent. I have even used text-to-speech software myself to edit my writing. Listening to a report I can catch errors that the speel check alone might miss. I have collegues that also use this tool for the same reason.
I was interested to see that the research showed little improvement in spelling ability. The article also stressed how all decisions for assistive technoloy be evaluated for eachusers specific needs.