I'm not sure how big the dictionary is, I hardly ever use that feature. It seems fairly complete, and it does learn from your spelling and grammatical habits to a certain extent. It might be ok if you are using a soft-keyboard, but taking thumbs off of the hardware keyboard doesn't work. The only time I pay it any attention is when it predicts a phrase or sentence that I need. Then it's worth the extra effort to select it.
For the most part, my typing speed on the desktop is sufficient that using such a feature would result in a net reduction of efficiency.
Granted, it would be useful for hard to spell words, but many common typo's are already auto-corrected. I do use autocorrect for a few simple things like transforming a small text tag into a large predefined snippet. Hmmm...Visual Studio style snippets might be cool for things like inserting an address. Of course, I would like to see Windows, or at least Office, implement some kind of microformats system.