I didn't harvest all my potatoes last year, I left some in the ground. They sprouted and have been growing slowly all summer. They look like they will be mature by the end of summer. Are two year old potatoes worth harvesting?

yes!! I have volunteers pop up in my garden too. You can dig them up early if you want new potatoes (the small ones) or wait until end of the summer. They should grow just as good as if you planted them this year on purpose.

Yes they are no different from planted potatoes.
Yield of volunteers is usually less than planted from my experience but I have very heavy ground and volunteers are many potatoes from year before in very close proximity and potatoes do not like to be crowded.