Opera was an innovative and customizable web browser. It created many features adopted by other browsers (e.g. tabbed browsing, gestures, extensive right-click menus among others). It never attained a large market share for a variety of reasons, none related the browser's functionality or reliability (see also: Beta videotape).
Opera's demise was a voluntary decision of the company to stop making a web browser, and instead create an overlay for Google's Chrome browser (similar to Rockmelt and Iron, or Maxthon based on Internet Explorer). It is no longer a browser, but instead an attempt to cut costs, an admission of defeat by a company that once led the field in innovative thinking.
There is question about Google's willing capitulation to dictatorial regimes (e.g. the People's Republic of China, the United States of America) in name of profit. The browser is known to have security issues, and it is unknown if (very likely that) software keys were turned over to governments, rendering users' data insecure and threatening one's privacy. The same insecurities are likely also true of Apple's browser, Safari.
The last version of Opera ever made was v12.16, which users should download and save. In the short term (the next year, perhaps two) v12 will suffice until technologies change. In the long term, migration to Firefox is the only option for those concerned about security. Firefox's functionality is less than what Opera 12's was (keyboard and mouse combinations, customizability of the keyboard), but is better than Opera 15.