So no, it's not 100% secure, but it's far far better than having an unsecured http:// connection.
As for the green lock, you can blame that on Chrome. I have no idea why they insist on using a green lock and green "Secure" text for DV certs. Safari only uses a green lock / green text for EV certs, which is a lot better (and I don't know offhand what Firefox or Edge do). Of course, you could have an EV cert and still use Flexible SSL, but anyone who cares enough to get an EV cert should know better than to use Flexible SSL anyway, and there's a great many ways to make your server insecure, using Flexible SSL is very far from the worst way.
All that said, it would be great if CloudFlare would just stop offering Flexible SSL in favor of the self-signed CSR approach. Any CloudFlare customer who can create their own cert to talk to CloudFlare can also create a CSR to get a cert from CloudFlare just as easily, so it's not clear to me why they still even offer Flexible SSL.
[1]: I thought Flexible SSL was the option to use an arbitrary self-signed cert on the origin server. gkop pointed out that, no, Flexible SSL means no encryption at all.
How is it secure? CloudFlare allows you to send this traffic in the clear. If they required this traffic be HTTPS, that would be far better for web security.
When observing non-technical users, I still see people clicking through blatant full page cert errors after connecting to WiFi because they've been implicitly trained that it's the captive portal making them sign in.