[bbromhead]

Great idea although it doesn't appear to pick up the misuse of there, their and they're.

I typed "I'm going to walk over their" and Nitpicker only told me that " I'm " is an informal contraction.

[ampersandy]

Overly picky, at the cost of comprehensiveness I would assume. :)

I might enjoy using this service if it was offered as a browser extension, since I find myself regularly proofing my writing to eradicate the informal language that I tend to use in my first drafts.