I'm not sure that this applies to all manufacturers but with Garmin, I found out that THEY determine when your "Lifetime" updates will expire. I had two Garmin units that were working fine. When I went to update them, the website said that they no longer will support that particular device and that new updates were not available. I was given the opportunity to exchange my two older units for two smaller, units that sold for a lot less than the two that I had that were now becoming obsolete. Garmin also offered a 20% discount on any other unit that I wanted to purchase, which basically dropped their price down to what I could buy the same unit at a big box store.
Basically, all manufacturers update their products on a regular basis and the older units become obsolete and are no longer in production. When this happens, you may find that altho your GPS unit is working fine but that you can no longer get updates, even with your LIFETIME updates. Read the fine print on your LIFETIME agreement and you will see what I mean.
Basically you may want to purchase the most recent GPS product instead of a discounted GPS that may be near the end of it's production run.
On the Garmin products, the LIFETIME of your GPS is defined as follows:
The Garmin Lifetime program allows you to receive up to four updates per year during the useful life of that GPS product, or as long as Garmin receives map data from it's current third party supplier, whichever is SHORTER.