Ok ive never used it, however wouldnt most of your clients be having sites hosted by someone else? Not sure on your thinking but I find most do and most hosts these days do have Zend optimizer installed.
As for clients downloading it Id say no, however you could do it and tell them exactly what it does, why your using it etc, most of the time if the client can tell you are being honest they will have no problems with allowing you to install it or download/install it themselves.