I started trying out Opera Mini today and so far I like it. I can see that sometimes the data saver feature makes loading websites "hang" a bit. It all depends on the website though, and on certain sites it works fine. I was thinking it might be a good idea to add a feature to allow turning the data saver off on specified domains. You could have a simple "blacklist" feature to turn off the data saver completely depending on the domain.
I was also thinking it might be interesting to have an option to automatically disable data saver if on wifi.