Yes, it's worth it for me. I've been waiting for the new iMac for over a year. Within the last month my needs have increased because my old one (early 2006) is significantly slower and I'm working on a masters thesis.
I almost bought the refurbished when the new one was announced, mostly because the improvements weren't worth the wait. I decided not to because they would be here in December.
Truth is, even if they do release in December, I might not even get one until February due to availability. If that's the case, I'm perfectly happy with the refurbished model. At least I know they are stable and are good machines.
If the new one is available in December I will go and check it out. If I think it's better than mine and worth the hassle, then I'll do the swap. I'm not just doing it to rent the computer for a month with 100% return intentions. I'm just hedging my bets a bit.