Interagency SOP's

With all the mutual aid being utilized these days, is anyone out there utilizing SOP's adopted by a mutual aid group or by numerous depts?. How is it working oout? What pitfalls or resistance was there?
Before regionalization into NHRFR, the 5 towns involved utilized alot of M/A and automatic aid. We shared a dispatching agency (North Hudson Regional Communications -- "Fire Control") and we shared IMS, Mayday, Rope-guided search, and were incorporating a mutual aid hi-rise SOP. We also did joint purchaisng of SCBA, radios, and turnouts among other things. There were others in the works, but the fiscal hand of regionalization swept in and the rest is history.