Ranked in these QuestionsQuestion Ranking
Pro Completely free
Pro Has built-in ad blocker
Pro Has a data saver
Opera mini's data saver has two modes - regular and extreme. great for people with smaller data plans when not on wifi. Has also the ability to control the quality of downloaded images and some video optimizations.
Pro Very light weight
Great for phones with lower RAM or performance issues.
Con Doesn't support some common modern browsers functionalities
Some web pages might look a bit broken but from personal experience it doesn't happen often enough (or at all) to be a major issue.