How useful is this camera without the seller provided storage? Does it work with my own server?

The Nest Aware won't work with your own personal server as it is saved to the cloud. You can still use the Nest Cam(s) without a subscription, but you would miss out on a few features: https://nest.com/support/article/What-do-I-get-with-Nest-Aware-for-Nest-Cam