Back in 2009 I bought a used projector on eBay (a Panasonic PT-AX100U) for $400 and the previous owner had bought a Mack extended warranty, which I was able to transfer into my name. If I recall, Mack charges a small fee for transfer of the warranty ($15?) but they agreed to waive it when I asked. Shortly thereafter I discovered that the projector had a problem with part of the screen being discolored. When I sent it into Mack, they determined that it was not worth repairing, and they issued me a check for $1000 to cover the cost of the replacement model (PT-AX200U).
Needless to say, I was extremely impressed, and ever since then I've been sharing my experience whenever I see people discussing Mack warranties.
After my good experience with the projector, I have purchased Mack warranties from some of my lenses. The Mack warranty prices are generally very reasonable, so it's not a big risk in the first place. Fortunately, I have not needed to use the warranty on any of my lenses. But then again, I don't own any Pentax lenses.