Do you think it makes sense to enforce that every member of a team must use the same IDE?

For instance all engineers that are already on the team use IDE X. Two new engineers come and want to use IDE Y instead because that's what they have been using for several years now.

Do you have any experience with "mixed IDE" teams? If so what is it?

Provided the 'official' build system (as used by the Continuous Build servers) is the same for all, I don't see any reason why each member of the team could not choose the tools he wants...