I think it's a good investment, particularly for the peace of mind it provides - I think of my 800MHz 17" iMac as "ancient", and yet it has over half a year left under warranty.
If you bought lots of Macs, you'd be better off not getting it, probably, and saving the saved money for repairs on the few that broke down. However, mine has paid off on my PowerMac already - fixed the 20" ACD that I bought with it.
Note that you can wait 90 days before buying it with NO repercussions, and a year if you're willing/able to do without free phone support after 90 days.
I'd especially recommend it for the iMac. For the mini, well, it's a pretty big percentage of the cost of the whole system. Unless you think there's a better than, say, 1 in 4 chance it'll break after a year and before three years, it probably isn't worth it.