My daughter used 800 many, many times with great results and no complaints. It has been awhile, tho. We have BOTH removed as much chemical interference in our pets and rely on more natural remedies for flee/tick/etc.
For the yard I use MOSQUITO BARRIER. Yes, it has garlic in it - but I have NOT noticed the dogs constantly licking the grass in order to ingest a dangerous or troublesome amount. The bugs are non existent immediately and the yard smelling like bread stix from olive garden isn't too terrible.
I do worry about heartworm. I have had them checked and they are negative. I was given Heartguard and it has remained in the cabinet.
I am one that feels medicines are temporary-and sometimes necessary. I also feel they are way overused on animals AND humans and the results are devastating. I found a very nice oil from Dr. Mercola to use on them if we find ourselves off the beaten path, just in case. I do use it on the underside of their bedding after washing, too. smells wonderful. (just do not get into eyes).
I am for this as well:
Nature's Sunshine Heartworm Prevention - Dog Health Report - Dog Care - Dog Health Problems Solved!