Shoutbox

I think that Opera is the best browser. I have been a paying user since version 6.

I think it is awesome, because right out of the box, it has so many features that would take hours of customisation to get in firefox.
Also, everything, and I mean everything is customisable. At a basic level, you can customize how the toolbars look, by moving stuff around, but you can also customise everything, and even create your own buttons. I have created my own customised version of Opera with a load of developer tools. For example, I can quickly check any page that I am on with one of five different validators, which is great for debugging. You can also code your own buttons, and mouse guestures if you want.

Also, Opera is very very innovative, it was the first browser to have tabs, first (and only) Windows browser to pass Acid2, Gmail is an exact copy of Opera's built in e-mail client (M2), first browser to have mouse guestures, only browser to have built in bittorrent support.
If you want to see what browsers of the future will look like, Opera is the way to go.

I have in the past tried to use Firefox, but I was too used to Opera's way of doing things so I switched back within a day.
I think its the small things that will make you switch back, for example, the straw that broke the camel's back for me was the fact that the tabs didn't resize depending on the length of the webpages' titles