Opera has trouble with some CSS style sheeting, as well as displaying some dynamic content pages. I've seen relatively beautiful, but uncompliant pages rendered in CSS absolutely butchered, but generally CSS support in Opera is pretty good.
It also has trouble sometimes with flash and 3rd party plug-in type pages. A case in point was when I was doing browser compatibility testing of my son's website: http://marclesemann.5u.com
When I test it with Firefox and IE it works beautifully...but when I tested it with Opera it concatenated some of the text, and changed the font. I turn the CSS off...and it was fine.
Opera, in my opinion, suffers from a case of the "information overloads" there are so many things and stuff to click, pull down, and otherwise do something with that when you get a clean, but highly customizable browser like Firefox you kinda take a breath of fresh air...not smog, like I do when I use any other "new" browser.
Firefox is what Netscape should be right now, simple, clean, well supported. I still prefer IE 6, but that's more a matter of habit. The more I use Firefox, the more I fall in love with it...I can't say that about Opera.
Opera has come a whole hell of a long way since the late 90's when it was introduced, but it's lost it's path along the way somewhere.