I only use HTTPS and a certificate for my ownCloud instance. There is only one real user of my ownCloud - me - so it's no big deal to have to continually tell Firefox and Chrome that it's okay, don't run away screaming from my website. But I wish I didn't have to.
If I had a lot of users, and they were paying money, I would get a certificate that would be accepted by the nanny browsers without question. Then, the $100 cost per year for a certificate might be worth it. But when there's only me, it's not worth it.
I've been reading about Let's Encrypt and they are planning on supplying free signed certificates. The public beta will be available on 3rd December. I will wait a bit to let the rush die down, then I will try it out. The certificates last 90 days, but the process of renewing them is automatic and can be added to cron.