I'm currently training for it, not necessarily to use it as my main qualification, but as a platform. Will then be training in Win 7 to enterprise level with in the next few months, and after that getting a job and carry on training (I love it, even if I never use it will carry on getting more and more qualifications).
Yes, it does open doors, however it depends what doors you want open. If you want to be first, second, third line support, field support or another support role in the IT industry, it will be a good way to get your foot in the door of some jobs.
I agree with the other two though, specialise and don't think being MS cert is enough