I have been doing massive Web Development targeting Chrome (Safari), Firefox and certainly IE.
I also give Opera a shot from time to time. My review is: Their rendering engine is fine, though not perfect. They have some minor problems. However, the JavaScript engine is awful. It doesn't behave like FF or Chrome engines. It's stupidly slow and sometimes freeze the browser.
Font rendering isn't optimal, still better than IE.
I work as a core QA at Opera. Do you have examples of the javascript problems that you experienced? We rewrote the engine for 10.50 so it's possible that that has fixed the issues you saw. On the DOM side we are working hard with other vendors through W3C / WHATWG / etc. to make the web specs precise enough that we can all converge on the same behaviour.
Any bugs that you report [1] would be much appreciated.
Funny, I do web development mainly in Opera and find that other browsers are fine, though not perfect :)
It's probably the same reason why people who develop mainly in IE find that Firefox/Chrome "is broken". Browser that you develop for is going to seem best, because during development you automatically adjust to its problems.
Opera was one of the first to pass Acid2 and render Acid3 pixel-perfect. They've been a bit late to the rounded corners/shadows game, but done them well (e.g. can render border style and radius on same element properly). They have really solid layout engine.
That "stupidly slow" JS engine is lately going head to head with V8.
One thing we can probably agree on is that JS error reporting in Opera is awful.
I also give Opera a shot from time to time. My review is: Their rendering engine is fine, though not perfect. They have some minor problems. However, the JavaScript engine is awful. It doesn't behave like FF or Chrome engines. It's stupidly slow and sometimes freeze the browser. Font rendering isn't optimal, still better than IE.