Firefox 4 has copied Opera 10.5 design (released 5 months ago) almost in its entirety. They just didn't use the "Opera" logo. I'd say every other aspect from the design has been copied. Just look at
Opera (design implemented 5 months ago) [picture 2] and the new and not-yet-released
Firefox 4 (design still to be implemented) [picture 2].
I use Opera on my PC and Opera Mobile on my Nokia N95. It syncs all of my bookmarks, speed dial pages, add-ons (you can have Opera Widgets on the PC and on the phone too, and they are just about the same), and makes my life a lot easier. I can browse a website on the phone and then when I get home quickly resume browsing it on my PC using Opera, and vice-versa.
I have already used the following web browsers: Internet Explorer (from 5 to 8), Firefox (from 2 to 4), Safari (latest only), Chrome (from first launch until latest version) and opera (from 8.x to latest 10.6 version).
Opera in my opinion is the best web browser. Not only the one that looks better, but also the fastest, easier to use and most user friendly. It is incredibly stable and allows you to use as many tabs as you want without any problems (I tried doing this with Firefox 3 and it crashes too often). Its design is also very comfortable.
The built-in mouse gestures (and Mobile gestures) make browsing the web faster and easier and requires no skills to use.
Opera is a browser but it actually allows you to do a lot more. The following features make Opera different from the other browsers (although these are not all of its features). It has built-in e-mail reader, built-in torrent manager (download torrents within the browser), built-in spell check in many (if not all) languages, Opera Unite (multiple uses: ability to use your pc as a host/server for your own website, for sharing files, pictures, songs or whatever you want directly from your pc to your friends without having to upload it anywhere, etc), allows add-ons and widgets, etc.
Anyway... in the end of the day, it's up to each person to decide which browser they should use. This is a personal opinion and every person may will find different features useful or useless according to their needs and their routine, so you should check out each browser's features and see which fits you best!