I've used it a couple years ago with version 4. I found it quite ergonomic. A lot a people didn't like the MDI but I found it nice especially when you could save all these when upon exiting. It's kind of like what they say about the VI editor: If you actually try to use it for more than 5 minutes, you'd come to love it.
However, for transactional type stuff, it's kinda of spotty. You of have to go back to IE for this, especially with Banks and the like. Also the settings for managing cookies, java, and scripting was "all or nothing".
IE is much better than Opera or Mozilla in this regard. Has that gotten since better since 4.X?
As others have said. Opera would be great if "everyone would just adhere to the standards". Unfortunately, that doesn't seem to be likely in the near future.
It's good if all you want to do is just read articles and forums.