There are 1108 Feliway reviews on Amazon. 66% of them award Feliway 4 or 5 stars. However 20% of them award only 1 or 2 stars, so it seems to work for most cats but not for a minority. (There are even some reports of it making the situation worse):
http://www.amazon.co.uk/Feliway-20066-Diffuser-Pack-48ml/dp/B0031TFZRK
So it might be worth a try but there can be no guarantees. (I've considered it myself for my nervous little boy but not yet tried it. The cheapest place on the High Street seems to be Asda pharmacies, where it's far cheaper than at most vets).