Well, first I think we need to agree that there are two different cases here: 1) interconnecting IXes operated by the same party, vs. 2) interconnecting IXes operated by different parties.
PAIX has successful implementations of both of these (I count our metro strategy as an instance of the first sort, and our varied interconnections to other switch fabrics operated by other people as an instance of the second). With both, as I stated earlier, a thorough understanding of the shortcomings of L2 fabrics as a place for different parties to meet is crucial to avoiding service-affecting outages. The lessons learned along the way are very helpful in debugging quickly and thoroughly when a service-affecting outage creeps through. Stephen VP, Eng. PAIX