Yes, I have used Feliway.  I kept 3 Feliway diffusers plugged in when I
adopted Duffy because Holly had always been very aggressive toward any other
cat that was in my vicinity.  I took a very slow, careful approach in
introducing them, and Feliway seemed to help a lot.  It is not a cureall,
but it did have a calming effect.  I no longer use it on a regular basis,
but I still keep some on hand in case something seems upsetting to my
furbabies -- for example, toward the end of construction when I had a
sunroom added.  They did very well until the last week, then began to seem
agitated.  I plugged in the Feliway, and there were no further problems.  I
also keep a can of the spray on hand, and I spray some in the cat carriers
about 20 minutes before I take them to the veterinarian.

As I said, it's not a cure-all, and some people on these newsgroups report
that it did not have any effect.  However, I think it is well worth trying.
The spray is good for occasional uses (such as a trip to the vet), but the
plug-in diffusers are better for continuous use because they release
premeasured amounts on a fairly constant basis.
Here is some information from an Ohio State University study on this type of
product: http://www.sciencedaily.com/releases/2000/11/001129074611.

MaryL

Photos of Duffy and Holly:      >'o'<
Duffy:  http://tinyurl.com/cslwf
Holly:  http://tinyurl.com/9t68o
Duffy and Holly together:  http://tinyurl.com/8b47e