Opera was in the past (versions 5.x and 6.x) relatively not used, but now with the versions 7.x Opera became one of the best browsers actually available.
Why would you choose Opera ?
+Simply because it's fast and small, it has many useful features (such as cookies and mouse gestures), it's standard compliant, and it's highly customizable. And best of all everything happens on runtime, i.e. skins, filters, plugins etc everything on the 'Fly'.
-Simply because it's fast and small, it has many useful features (such as cookies and mouse gestures), it's standard compliant, and it's highly customizable.