Well, Opera 7 is cool.
And it can display Quadrone (http://www.quadrone.org/ - my favourite "IE sux" page) properly (alright, with minor errors)

But, there are some things I prefer Mozilla for:
1. Mozilla (Firebird) is opensource and completely free (no adds, no toolbar space and bandwidth wasted)
2. Mozilla (Firebird) is fully configurable via about:config
3. Mozilla is made for web developers (it comes with useful web developer tools)
4. Mozilla uses cross platform language called XUL - Opera is available, but it's not same codebase for all platforms
5. Mozilla nightlies are compiled every day - easy bug fix tracking
6. Mozilla Firebird has dozens of various extensions
7. Smoothscrolling is not present in Opera - IE has it (and it's best out there), Mozilla has it (it's not so good, but not bad)
8. Some pages I usually visit don't display properly in Opera (and they display properly in both IE and Mozilla)
9. Indeed, Opera is faster, but I personally don't like the way it loads pages (loading styles after creating page looks awful to me) - Mozilla handles it better
And perhaps some others I can't currently think of.
Opera is cool, interface is good looking (especially if you install Qute clone for Opera:)) and useful, but, for me, it can't beat Mozilla Firebird.
As somenoe said on MozillaZine forums:
QUOTE
Opera pushes some Mozilla people to make a fast browsers (such as Phoenix), Mozilla pushes Opera to increase their support for the standards. As long as they push each other to make better browsers I am happy.