It makes me wonder what kind of sites aren't compatible with Opera, as I've only ever come across one (www.whereis.com.au). What are some with known issues?
Right-click when in the whereis.com.au page you're taken to. "Edit Site Preferences". In the Network tab, change the Browser Identification to "Identify as Internet Explorer". It is just bad coding on their part to restrict content to particular browsers. I use it all the time with Opera with no issues.
i love firefox, but opera is also good. firefox offers a better protection and is still fast to browse.
firefox is the primary. i use opera to always have access to sites i need to reach fast and explorer only if i need a software to run with it, by example indexa stuff.