I'm another satisfied Feliway user!
I've used it successfully for integration issues and also a urinating issue that was behavioral and not health related.
The spray is good for stopping inappropriate spraying or marking with urine and also for spraying on towels that go in the carrier during vet visits.
The plug-ins are great for integrations, keeping cats who are more high-strung calmer, etc.

It's always a good idea, when taking one pet of a multi-pet household, to take a towel and rub it on all the pets NOT going to the vet, put that in the carrier with the pet who IS going to the vet...use that towel to rub the pet down with real well before bringing them back into the home.