SQL Server vs MongoDB?

I have been happily using SQL Server for over 5 years and am convinced of the power of the relational model.

However, next week I will be meeting with some folks who are hooked on MongoDB and not in favour of using an RDBMS (of any flavour.)

I want to avoid a "religeous wars" type of discussion so in the last few days, I have trawled the web and read a MongoDB book but it is not easy to find hard facts about the detailed issues relating to "RDBMS or MongoDB" ?

So it would be nice to get some clues as to the "hard" and "non-hype" answers to the questions:

1. Under what circumstances is SQL Server to be preferred to MongoDB?
2. Under what circumstances is MongoDB to be preferred to SQL Server (2008 or 2008 R2)

Has anyone any comparative experience of the two alternative solutions?
And how would you approach such a discussion?

Thanks

Ken