I do not have any personal experience, but I'm sure some others will be able
to provide personal insight.  However, here are several sites (with photos)
that you might want to review.  Remember, many of these will keep your cats
*in* but may not keep other cats *out* (but then they could be trapped in
your yard).  So, you should probably make provisions for that by installing
similar safeguards on the other side of the fence if there is any way for
neighborhood cats to get past the barrier and into your yard.
Design for do-it-yourself barrier to mount on top of fence (to keep cats
in):
http://www.lisaviolet.com/cathouse/backyard.html

Outdoor fence enclosure (several views):
http://www.just4cats.com/
http://www.just4cats.com/post1.html
http://www.just4cats.com/page7.html

A guide to inexpensive do-it-yourself fencing:
http://www.feralcat.com/fence.html

Cat fencing US mail order.
http://www.catfencein.com/ http://www.fabcats.org
(includes an information sheet you can download)

"Friendly Fence": web site says it is virtually invisible
www.friendlyfence.com Other ideas for outdoor fencing for cats:

http://www.just4cats.com/page7.html
http://www.cat-world.com.au/cat-worldenclosures.htm
http://www.lisaviolet.com/cathouse/backyard.html

KittyKlips -- addition to existing wood fence to prevent cats from climbing
[note: but would not prevent cats on "other" side of fence from getting in]:
http://kittyklips.com/details.htm

Good luck!
MaryL

I have the Fence-In system installed around my patio. The version I have
keeps cats out as well as in.  It has worked beautifully on the fence for
over a year, and my colony of ferals has not made it across.  I'm still
having occasional trouble with a tree though and one cat who gets through
the netting from time to time. He doesn't want to go anywhere. He just
sits on top of the fence and laughs at me. :(

BarB