I didn't buy one for my Roadster. Had it out of warranty for four years. Glad I didn't buy one; I saved a lot of money that way.
Our Signature Model S was out of warranty for 3 years. No warranty was the way to go there too.
Haven't had any of the more recent cars out of warranty yet...
As whitex says, it depends on whether your out-of-warranty expenses are more or less than what Tesla charges for the warranty. It will be more for some, less for others, and there is no way to know in advance which group you will be in. So I view it like insurance...if you can't afford to fix/replace it on your own, you should buy insurance. If you can afford to fix/replace it on your own, financially you are likely to be better off self-insuring...but if worries about future expenditures keep you up at night, it can still be worth getting for the peace of mind.