Check Consumer Reports. Even on fairly expensive electronics they are a bad bet (remember, you're betting it will fail and Apple, with more info, is betting it won't).
Also, check your credit card. Mine extends the warranty a year; why pay twice for that same year of coverage?
The failure rate with iMacs after a year just isn't that high. And I suspect a bunch of returns aren't really hardware issues. And the Apple warranty doesn't cover theft, dropping it, etc.
The rather unfortunate thing about iMacs, however, is that probably the more failure prone item is the hard drive, and they are ridiculously difficult to replace internally. However, with today's Tbolt you could operate with an external SSD at almost the same speed; most users wouldn't see a difference.
I dunno what your financial priorities are, but I'd put the money to some other use. We used the money we saved for a backup machine. I dunno if you just use it for play, but for us the loss of use while it sat at Apple repair was more of a money loser than the hardware itself.