Opera Pros:
1) It is fast.
2) It's small w/o the Java plugin (2 Mb±).
3) It has built-in News and Mail clients.
4) It pretty much abides by the HTML standard.
5) It has a lot of nice customization features.
Opera Cons:
1) It will cost you $$ to register unless you're willing to put up with some adware.
2) It has built-in News and Mail clients.

3) It pretty much abides by the HTML standard.

4) It's not small w/ the Java plugin (9 Mb±).
Many Opera users would prefer that the News and Mail clients be optional. They (we) just want a speedy browser, not a everything-but-the-kitchen sink NS/IE clone. It's my personal view that Opera will lose some of its user base because of this. Although, I expect it's Opera Software's view that the net gain in users will be positive, and they very well may be right.
Many web pages you'll come across display fine in IE, but when viewed in Opera, are garbled. This is an HTML standards issue which I won't go into. Suffice it to say, for an Opera user, it can be annoying.
It's good enough though, that I use it 95% of the time. When it runs across a page that doesn't display or function correctly, I open up IE.
This is probably more than (any of) you wanted to hear, but there it is.
