I have had a Zumo 550 for 4.5 years and a lifetime update for 3 years. I have had no problems with the downloads but you do have to download a plugin first. You also need to have the Zumo plugged in to your computer so as the download will detect it and authenticate the update is for your unit and update the maps on it. You have to couple your unit with the download on first update. You cannot down load your update to another unit or transfer your update to another unit once you have aligned it to a unit.
I have just bought a new Nuvi for my wife and this had the lifetime update free with it and I set it up no problem after downloading the plug in first.
Hope this helps Mike