It will be interesting to see how this plays out. The application, that I know of, has not had any real updates for several years and I have heard that it has not been a big seller so I don't see this as having much of an impact other than to get people in the open source and/or anti-MS world excited for a while.
I downloaded the application and it is true that the UI has not had a facelift since the first Clinton Administration.
The more I think about this the less impressed I am with it as a benefit to customers or as a threat to Microsoft Project. Microsoft has moved it's focus to the full enterprise with Project Server and Project Professional. Integration with a server based repository with reporting and time collection is a key part of the enterprise appeal of solutions like Project Server. Open Workbench is standalone and does not appear, at least at this point, to have any integration with a server based tool. (Though it would seem that this would be forthcoming)
So Open Workbench sits in an odd place. It is standalone which limits is utility within an Enterprise context which puts it in a similar place as Project Standard from Microsoft. But the InfoWorld article puts Workbench in a class for elite schedulers. Project does well because of it's relative ease. A product for elite users will likely disappoint and frustrate entry level or casual users. Even easy to use Project Standard causes new users fits when first coming to the product. And this is given the fact that there are TONS of books about Project and how those new to the product can use it. I have never seen a Workbench book aimed at newbies.
At this point I don't see a clear benefit except for high end schedulers that are interested in Workbench (and do not need to share information about their schedules with others) but did not have the $10K to buy it before.
That said it is obviously too early to tell what this really means for customers or for the market place.