Opera is an excellent piece of proprietary software.
Put that all together with a company which is clearly committed to producing a quality product instead of megalomania, and it's really hard to build a case against Opera, aside from it being under a proprietary license with no source code available.
- It follows open standards.
- There is no file format lock-in; configuration files, bookmarks, etc are plaintext.
- It is available for multiple hardware platforms and operating systems.
- It is well-supported with newsgroups and a bug tracker.
- It is reasonably priced proportional to the competition.
- With each upgrade comes significant new features.