I haven't used an IF personally, but I worked at Petsmart and met lots of people that did use them. A lot of the IF got returned because they were defective, so keep that in mind. Also, it generally isn't a good idea for a large amount of property. In your situation I suggest you put a physical fence around the perimeter of your property and then maybe use the radar based IF to keep the dogs near the house. Like others have said, an IF won't keep others off your property and often doesn't work well for dogs with a high pain tolerance (like Labs).
Another thing to remember is the collar is using electricity to correct your dog. While working in the salon at Pmart I saw a couple dogs who wore bark/IF collars and they literally had holes burned into their necks. If you do use one it can't be on at all times.