Nicely observed kaled ;)
Well I've been using Opera since v3.x (98-99) simply because I was looking for a IE alternative on Win. At that time on linux I believe I was using Mozilla, never had any problems with it.
First thing that grabbed my attention was the speed, slowly but surely I was stepping away from IE. Not to forget "Transfers" back in the day on 56K's pausing your download meant salvation. Later on I officially set Opera as my default on both win and nix.
Opera went even beyond my needs, and every version brought some major changes.
When I installed FireFox, (as an Opera user) it terribly bothered me that new window was actually opened in new window! Tabbed browsing was nothing like compared to Opera. Surfing was nothing special, switching and opening new tabs was messed up. Download manager was ... too miniature ...
Opera, everything! It saves my time, money, (yes money!), workspace, nerves, opera saves the day ;) These guys really know what you need. Development team is great, and if you have any suggestions you can post in the wish-list and it actually get's included in one of the future versions. Now people will say it's not free and whatever. What you pay is what you get!
Well I dont think FireFox is a great browser, it's just something IE "fans" needed, download manager, tabbed browsing may seem fascinating to you if all you ever used was IE. That is why it got so many fans out of a blue. And ahh yes it's free. It only proves how much the market needs great free (open source?) programs, to be particular windows users.
Basically it's simple as this:
Novice user: Use IE
With IE I feel like I'm naked, floating like an astronaut in this www space.
Intermediate User: Use FireFox
Well it just feels like driving "Yugo" and yes I'll get "BMW" one day.
Advanced User: Use Opera
Using Opera I feel like I'm navigating this invincible jet-alike tank, fast, safe and sound. Within few clicks I'll go through 20 pages, read my email, I'll even get to write few sticky notes :P