About friggin' time.
This is something that will definitely benefit the community. It will help sway people on the fence about GNU/GPL stuff such as myself to warm up a bit to the nasty software giant.
But will it benefit Microsoft? Joel Spolsky states that the Office formats are overly complicated and that anybody attempting to clone the software definitely has their work cut out for them. It is a valid point, however the open-source community is capable of doing very difficult things, and this is an excellent opportunity to make them not suck so much. This means that Microsoft will have some competition, something they probably don't want.
It will also benefit Microsoft. Releasing their specifications will mean that more people will develop software using their formats, further spreading the Microsoft platforms. People will develop alternative software to Microsoft's versions, meaning that the quality of Windows applications will in general improve, which may help to stem the growth of Mac and Linux.
I guess we'll just wait and see.