I'm pretty sure that you can get a much better deal if you purchase the warranty through Ziller as opposed to responding to the Generac mailing. It's the exact same Generac warranty. Generally I'm not a big fan of extended warranties but looking at the pricing of some of the parts it could be a good investment.
I bought the extended warranty through Ziller. I'm very comfortable with electronic troubleshooting and repair. Internal combustion engines not so much. Add in dealer labor costs and it seems like a good insurance policy. Another factor would be the number and duration of typical power failures in your area. If you have frequent outages of several hours, your generator will see a lot more usage and be more likely to need service.