Back when they still had their own browser engine it was the greatest browser at the time. HTML5 was rolling in hard and Opera was always the first one to implement these new features. It was also faster than any other browser, had customizable UI (with full MDI instead of just tabs), builtin E-mail client and good tools for Web developers.
But as an open source person using it always felt a little bit wrong, because of it’s closed source nature. Now that it’s just an alternative UI for Chrome and owned by some shady Chinese company I wouldn’t touch the damn thing with thousand foot pole.