This page used to have a long FAQ that we thought was useful when Eigen was LGPL-licensed.
Now that Eigen is MPL2-licensed, there is no need anymore for that. Common questions about the MPL2 are answered on the MPL2 FAQ. Do not hesitate to write to the Eigen mailing list if you have further questions, although we aren't lawyers and you will be better served by asking directly the MPL2 specialists (see above MPL2 FAQ link).
The MPL2 is a simple, file-level-copyleft license (meaning that the copyleft does not propagate across source file boundaries) which should allow use by all projects.