Opera is an interesting browser, that was first on the scene for many advanced features in the browser (tabs, zooming, security), but didn't gain a lot of traction because it was distributed on a "shareware"-like model when competing directly against IE. It's since been offered for free (since Firefox became very popular), and has had a significant increase in users.
Its fast, blazing fast!
Has every feature you'll ever need
Follows most spec items very well
Has a legacy of mimicking IE, including many of IE's bugs
Doesn't allow [Tab] navigation of hyperlinks :-(
Bugs: (bug 152)
More to come...